FreeBSD, glibc, and musl have all implemented C11 threads at this
point. POSIX is looking at how to align with it. Probably time to jump
on the bandwagon ourselves...
Implemented in the same style as <termios.h> so we can provide this
functionality even on older API levels. This does mean that this is
strictly more expensive than calling pthread functions directly.
Although this isn't in POSIX yet, assume that it's going to be basically
the same as C11 and add the header test anyway. We should revisit this
when POSIX actually adds <threads.h>.
Test: new tests
Change-Id: I8602d67ce71ca7f8ed1529daa0a8ea1feb083dd6
