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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

sys/sys: further 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.

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

Remove _SOLARIS_C_SOURCE compatibility definitions. Unfortunately the
ZFS porting style didn't extend this, instead using a heap of additional
header files that don't get installed.

My intention had been to allow OpenSolaris external code to build on
FreeBSD out of the box (i.e. without a src tree).

This file is the same as the one installed in /usr/include, however
there are enough places in the DTrace kernel/module sources that
having a header that gathers together all the individual elf headers
is convenient.

Note that the Solaris compatibility definions are conditionally
included iff _SOLARIS_C_SOURCE is defined.