Move mutex, condvar, and thread-specific data routes, pthread_once, and
pthread_exit from libpthread to libc, along with low-level bits to
support them. Major bump to both libc and libpthread.

Requested by libressl team. Ports testing by naddy@
ok kettenis@

Copy files from ../librthread in preparation for moving functionality
from libpthread to libc. No changes to the build yet, just making it
easier to review the substantive diffs.

ok beck@ kettenis@ tedu@