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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

linux: For better compatibility, provide compatible endian.h

Add endian.h. This includes sys/endian.h and then adds extra defines
that glibc defines with double underscores for our
_{BIG,BYTE,LITTLE,PDP}_ENDIAN macros. We also define __FLOAT_WORD_ORDER
to be the same as _BYTE_ENDIAN since FreeBSD doesn't currently define
this, and the default with glibc is exactly this for our platforms.
Move common parts of endian.h and sys/endian.h into sys/_endian.h
to limit namespace pollution from endian.h

All this gives us good compatibility with Linux. There may be one or two
upstreams that haven't integrated the patches I tried to send up.

There are some minor differences:
o The extra glibc macros are not defined. These are all
controlled with either __ at the start, or only defined
when glibc is being built. We also don't define macros
that are used internally in glibc that would pollute
the namespace.
o For complete compatibility, this change must also be
paired with providing a glibc-compatible byteswap.h.

Sponsored by: Netflix
Reviewed by: mhorne, markj, jhb
Differential Revision: https://reviews.freebsd.org/D31962

endian.h: Use the __bswap* versions

Make it possible to have all these macros work without bswap* being
defined. bswap* is part of the application namespace and applications
are free to redefine those functions.

Reviewed by: emaste,jhb,markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D31964

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

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.

Fix some way-past-brucification complaints from FlexeLint.

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

Quiet warnings about mis-matched pointer assignment.

Tested on: PowerPC

Pointy hat commit:

Don't Cut&Paste from big endian to little endian function without subsequent
adjustments.

Use "unsigned char" instead of "u_char" to avoid trouble.

Add inline functions {be,le}{16,32,64}{enc,dec}() for encoding decoding
into byte strings of unknown alignment.

Make this header self-reliant with regard to the types it uses.

Move the new byte order function prototypes from <sys/param.h> to
<sys/endian.h>. This puts us in line with NetBSD and OpenBSD.