Remove $FreeBSD$: one-line .h pattern

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

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@