sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

Pedantic polishing of code to please FlexeLint.

Approved by: ed

sys: general 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.

No functional change intended.

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

Fall back to ASCII codepoints for box drawing.

Even though the default VGA font provides box drawing fonts, there is no
guarantee any font will provide these as well (i.e. ISO-8859-*, KOI8-R).
Just use ASCII characters for box drawing.

PR: kern/141633

Get rid of now deprecated SCS wrappers.

We always build SCS, even when processing 8-bit data. There is no reason
why we should be able to disable it now.

Add __unused.

It turns out my previous commit does survive a buildkernel, but not
compilation of the individual test tools that use WARNS=6.

Make SCS work in 8-bit mode.

This means we can finally do things like VT100 box drawing when using
Syscons (8-bit characters). As far as I know, the only remaining issue
is the absense of proper escape sequences for special keyboard
characters (cursor, F1 to F12, etc) and xterm emulation should be ready
for general use.

Enabling xterm would have the following advantages:

- Easier possible migration to Unicode. cons25 termcap entries are very
8-bit centric. They use things like CP437 characters for box drawing,
etc.

- Better support for SSH'ing to other operating systems/devices. Most
switches use VT100-style admin interfaces.

- Reduced bandwidth, because applications can now use things like
scrolling regions.

- You can finally use applications like dtach(1) on both the console and
inside an xterm.

Move libteken out of the syscons directory.

I initially committed libteken to sys/dev/syscons/teken, but now that
I'm working on a console driver myself, I noticed this was not a good
decision. Move it to sys/teken to make it easier for other drivers to
use a terminal emulator.

Also list teken.c in sys/conf/files, instead of listing it in all the
files.arch files separately.