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

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

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.

Renumber copyright clause 4

Renumber cluase 4 to 3, per what everybody else did when BSD granted
them permission to remove clause 3. My insistance on keeping the same
numbering for legal reasons is too pedantic, so give up on that point.

Submitted by: Jan Schaumann <jschauma@stevens.edu>
Pull Request: https://github.com/freebsd/freebsd/pull/96

POSIX.1-2008 moved some constants from the XSI option to the Base.

MFC after: 2 weeks

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.

Define MQ_PRIO_MAX, it seems it is the only place
the definition will be shared by kernel and userland.

/* -> /*- for license, minor formatting changes

Remove advertising clause from University of California Regent's license,
per letter dated July 22, 1999.

Approved by: core

Move GID/UID_MAX under __BSD_VISIBLE protection.

Requested by: bde
Approved by: bms(mentor)

Tweak existing header and other build infrastructure to be able to build
pf/pflog/pfsync as modules. Do not list them in NOTES or modules/Makefile
(i.e. do not connect it to any (automatic) builds - yet).

Approved by: bms(mentor)

sys/sys/limits.h:

- Fix visibilty test for LONG_BIT and WORD_BIT. `#if defined(__FOO_VISIBLE)'
is alays wrong because __FOO_VISIBLE is always defined (to 0 for
invisibility).

sys/<arch>/include/limits.h
sys/<arch>/include/_limits.h:

- Style fixes.

Submitted by: bde
Reviewed by: bsdmike
Approved by: re (scottl)

Style fixes.
Remove DBL_DIG, DBL_MIN, DBL_MAX and their FLT_ counterparts, they
were marked for deprecation ever since SUSv1 at least.
Only define ULLONG_MIN/MAX and LLONG_MAX if long long type is
supported.
Restore a lost comment in MI _limits.h file and remove it from
sys/limits.h where it does not belong.

Properly define CHAR_MIN/CHAR_MAX taking into account whether or
not __CHAR_UNSIGNED__ is defined.

Fix a typo in UINT_MAX definition.

Add a new sys/limits.h file which in turn depends on machine/_limits.h
to get actual constant values. This is in preparation for machine/limits.h
retirement.

Discussed on: standards@
Submitted by: Craig Rodrigues <rodrigc@attbi.com> (*)
Modified by: kan