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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

kern: clean up empty lines in .c and .h files

Remove duplicated empty lines from kern/*.c

No functional changes.

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.

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

MFC 199549, 199997, 204158, 207673, and 208901.

Bring in a number of netfront changes:

r199549 | jhb

Remove commented out reference to if_watchdog and an assignment of zero to
if_timer.

Reviewed by: scottl

r199997 | gibbs

Add media ioctl support and link notifications so that devd will attempt
to run dhclient on a netfront (xn) device that is setup for DHCP in
/etc/rc.conf.

PR: kern/136251 (fixed differently than the submitted patch)

r204158 | kmacy

- make printf conditional
- fix witness warnings by making configuration lock a mutex

r207673 | joel

Switch to our preferred 2-clause BSD license.

Approved by: kmacy

r208901 | ken

A number of netfront fixes and stability improvements:

- Re-enable TSO. This was broken previously due to CSUM_TSO clearing the
CSUM_TCP flag, so our checksum flags were incorrectly set going to the
netback driver. That was fixed in r206844 in tcp_output.c, so we can
turn TSO back on here.

- Fix the way transmit slots are calculated, so that we can't overfill
the ring.

- Avoid sending packets with more fragments/segments than netback can
handle. The Linux netback code can only handle packets of
MAX_SKB_FRAGS, which turns out to be 18 on machines with 4K pages. We
can easily generate packets with 32 or so fragments with TSO turned on.
Right now the solution is just to drop the packets (since netback
doesn't seem to handle it gracefully), but we should come up with a way
to allow a driver to tell the TCP stack the maximum number of fragments
it can handle in a single packet.

- Fix the way the consumer is tracked in the receive path. It could get
out of sync fairly easily.

- Use standard Xen ring macros to make it clearer how netfront is using
the rings.

- Get rid of Linux-ish negative errno return values.

- Added more documentation to the driver.

- Refactored code to make it easier to read.

- Some other minor fixes.

Reviewed by: gibbs
Sponsored by: Spectra Logic

Approved by: re (bz)

Switch to our preferred 2-clause BSD license.

Approved by: kmacy

- bump __FreeBSD version to reflect added buf_ring, memory barriers,
and ifnet functions

- add memory barriers to <machine/atomic.h>
- update drivers to only conditionally define their own

- add lockless producer / consumer ring buffer
- remove ring buffer implementation from cxgb and update its callers

- add if_transmit(struct ifnet *ifp, struct mbuf *m) to ifnet to
allow drivers to efficiently manage multiple hardware queues
(i.e. not serialize all packets through one ifq)
- expose if_qflush to allow drivers to flush any driver managed queues

This work was supported by Bitgravity Inc. and Chelsio Inc.