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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

lib: 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.

Add lock annotations to the header files of our threading libraries.

This change extends all of the functions present in the <pthread.h> and
<threads.h> headers to have lock annotations. This will allow Clang to
warn about the following:

- Locking a function twice,
- Unlocking a function without a mutex being locked,
- Forgetting to unlock a mutex before returning,
- Destroying or reinitializing a mutex that is currenty locked,
- Using an unlocked mutex in combination with a condition variable.

Enabling these annotations already allowed me to catch a bug in one of
our userspace tools (r270749).

Add libstdthreads.

This library implements the C11 threads interface on top of the pthreads
library. As discussed on the lists, the preferred way to implement
this, is as a separate library.

It is unlikely that these functions will be used a lot in the future. It
would have been easier if the C11 working group standardized (a subset
of) pthreads and clock_nanosleep(). Having it as a separate library
allows the embedded people to omit it from their system.

Discussed on: arch@, threads@