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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

include: General further adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using mis-identified 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.

Fix <uchar.h> in for C++11.

It turns out that in C++11, char16_t and char32_t are built-in types;
language keywords. Just fix this by putting traditional _*_T_DECLARED
blocks around the definitions. We'll just predefine these in
<sys/_types.h>.

This also opens up the possibility to define char16_t in other header
files, if ever needed (e.g. if we would gain a <ctype.h> for
char16_t/char32_t).

Add <uchar.h>.

The <uchar.h> header, part of C11, adds a small number of utility
functions for 16/32-bit "universal" characters, which may or may not be
UTF-16/32. As our wchar_t is already ISO 10646, simply add light-weight
wrappers around wcrtomb() and mbrtowc().

While there, also add (non-yet-standard) _l functions, similar to the
ones we already have for the other locale-dependent functions.

Reviewed by: theraven