sys: Automated cleanup of cdefs and other formatting

Apply the following automated changes to try to eliminate
no-longer-needed sys/cdefs.h includes as well as now-empty
blank lines in a row.

Remove /^#if.*\n#endif.*\n#include\s+<sys/cdefs.h>.*\n/
Remove /\n+#include\s+<sys/cdefs.h>.*\n+#if.*\n#endif.*\n+/
Remove /\n+#if.*\n#endif.*\n+/
Remove /^#if.*\n#endif.*\n/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/types.h>/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/param.h>/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/capsicum.h>/

Sponsored by: Netflix

sys: Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

Minor style cleanup

We prefer 'while (0)' to 'while(0)' according to grep and stlye(9)'s
space after keyword rule. Remove a few stragglers of the latter.
Many of these usages were inconsistent within the file.

MFC After: 3 days
Sponsored by: Netflix

tty: use __unused annotation instead to silence warnings

tty: conditionally assign to ret value only used by MPASS statement

sys/kern: adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using misidentified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

Check tty_gone() after allocating IO buffers. The tty lock has to be
dropped then reacquired due to using M_WAITOK, which opens a window in
which the tty device can disappear. Check for this and return ENXIO
back up the call chain so that callers can cope.

This closes a race where TF_GONE would get set while buffers were being
allocated as part of ttydev_open(), causing a subsequent call to
ttydevsw_modem() later in ttydev_open() to assert.

Reported by: pho
Reviewed by: kib

Fix whitespace inconsistencies in the TTY layer and its drivers owned by me.

A new jail(8) with a configuration file, to replace the work currently done
by /etc/rc.d/jail.

Remove statistics from the TTY queues.

I added counters to see how often fast copying to userspace was actually
performed, which was only useful during development. Remove these
statistics now we know it to be effective.

Use unsigned longs for the TTY's sysctl stats.

Spotted by: clang

Slightly improve the design of the TTY buffer.

The TTY buffers used the standard <sys/queue.h> lists. Unfortunately
they have a big shortcoming. If you want to have a double linked list,
but no tail pointer, it's still not possible to obtain the previous
element in the list. Inside the buffers we don't need them. This is why
I switched to custom linked list macros. The macros will also keep track
of the amount of items in the list. Because it doesn't use a sentinel,
we can just initialize the queues with zero.

In its simplest form (the output queue), we will only keep two
references to blocks in the queue, namely the head of the list and the
last block in use. All free blocks are stored behind the last block in
use.

I noticed there was a very subtle bug in the previous code: in a very
uncommon corner case, it would uma_zfree() a block in the queue before
calling memcpy() to extract the data from the block.

- Import the HEAD csup code which is the basis for the cvsmode work.

Fix some edge cases in the TTY queues:

- In the current design, when a TTY decreases its baud rate, it tries to
shrink the queues. This may not always be possible, because it will
not free any blocks that are still filled with data.

Change the TTY queues to store a `quota' value as well, which means it
will not free any blocks when changing the baud rate, but when placing
blocks back into the queue. When the amount of blocks exceeds the
quota, they get freed.

It also fixes some edge cases, where TIOCSETA during read()/
write()-calls could actually make the queue a tiny bit bigger than in
normal cases.

- Don't leak blocks of memory when calling TIOCSETA when the device
driver abandons the TTY while allocating memory.

- Create ttyoutq_init() and ttyinq_init() to initialize the queues,
instead of initializing them by hand. The new TTY snoop driver also
creates an outq, so it's good to have a proper interface to do this.

Obtained from: //depot/projects/mpsafetty/...

Remove unused tty_gone() checks inside ttyoutq_read_uio().

When my earlier MPSAFE TTY prototypes still implemented line
disciplines, we needed a mechanism to abort read()'s on PTY master
devices when inside the line discipline. Because this is no longer the
case, these checks have become unneeded.

Integrate the new MPSAFE TTY layer to the FreeBSD operating system.

The last half year I've been working on a replacement TTY layer for the
FreeBSD kernel. The new TTY layer was designed to improve the following:

- Improved driver model:

The old TTY layer has a driver model that is not abstract enough to
make it friendly to use. A good example is the output path, where the
device drivers directly access the output buffers. This means that an
in-kernel PPP implementation must always convert network buffers into
TTY buffers.

If a PPP implementation would be built on top of the new TTY layer
(still needs a hooks layer, though), it would allow the PPP
implementation to directly hand the data to the TTY driver.

- Improved hotplugging:

With the old TTY layer, it isn't entirely safe to destroy TTY's from
the system. This implementation has a two-step destructing design,
where the driver first abandons the TTY. After all threads have left
the TTY, the TTY layer calls a routine in the driver, which can be
used to free resources (unit numbers, etc).

The pts(4) driver also implements this feature, which means
posix_openpt() will now return PTY's that are created on the fly.

- Improved performance:

One of the major improvements is the per-TTY mutex, which is expected
to improve scalability when compared to the old Giant locking.
Another change is the unbuffered copying to userspace, which is both
used on TTY device nodes and PTY masters.

Upgrading should be quite straightforward. Unlike previous versions,
existing kernel configuration files do not need to be changed, except
when they reference device drivers that are listed in UPDATING.

Obtained from: //depot/projects/mpsafetty/...
Approved by: philip (ex-mentor)
Discussed: on the lists, at BSDCan, at the DevSummit
Sponsored by: Snow B.V., the Netherlands
dcons(4) fixed by: kan