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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

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

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.

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

Add ng_tcpmss node, which alters MSS options of TCP packets. Useful to
hack MSS of packets outgoing via interface with small MTU, to workaround
path MTU discovery problems.

Written by Alexey Popov, with some cleanups from me. There are also plans
to improve mpd port, so that it uses this node, instead of doing MSS
hacking in userland, when 'enable tcpmssfix' option is on.

Submitted by: Alexey Popov <lollypop@flexuser.ru>