sys: Remove ancient SCCS tags.

Remove ancient SCCS tags from the tree, automated scripting, with two
minor fixup to keep things compiling. All the common forms in the tree
were removed with a perl script.

Sponsored by: Netflix

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

Remove /^\s*\*+\s*\$FreeBSD\$.*$\n/

Consolidate machine/endian.h definitions

This change serves two purposes.

First, we take advantage of the compiler provided endian definitions to
eliminate some long-standing duplication between the different versions
of this header. __BYTE_ORDER__ has been defined since GCC 4.6, so there
is no need to rely on platform defaults or e.g. __MIPSEB__ to determine
endianness. A new common sub-header is added, but there should be no
changes to the visibility of these definitions.

Second, this eliminates the hand-rolled __bswapNN() routines, again in
favor of the compiler builtins. This was done already for x86 in
e6ff6154d203. The benefit here is that we no longer have to maintain our
own implementations on each arch, and can instead rely on the compiler
to emit appropriate instructions or libcalls, as available. This should
result in equivalent or better code generation. Notably 32-bit arm will
start using the `rev` instruction for these routines, which is available
on armv6+.

PR: 236920
Reviewed by: arichardson, imp
Tested by: bdragon (BE powerpc)
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D29012

sys: further adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 3-Clause license.

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.

Special thanks to Wind River for providing access to "The Duke of
Highlander" tool: an older (2014) run over FreeBSD tree was useful as a
starting point.

ARM: Remove trailing whitespace from sys/arm/include
No functional changes.

Approved by: kib (mentor)

trim trailing whitespace

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.

Whitespace nit.

In __bswap16_var(), make sure the 16 upper bits are cleared; while
optimizing, gcc4 doesn't always do so.

Reported by: Nathan Whitehorn
Approved by: re (blanket)

Use a + constraint modifier for a register arg in __bswap16_var().

Reviewed by: cognet

Asm version of bswap16().

Obtained from: NetBSD

Start to support the big endian case as well.

Start all license statements with /*-

Use casts to enforce the return type of bswap16() and bswap32().

Add optimized version of the bswap macroes for constants if __OPTIMIZED__ is
defined.

*blush*
Fix htonl and htons.

Add some endianess-related functions and macros.

Style sync.

StrongARM platform-specific definitions.