Revert recent changes to unbreak ports/net/samba

While it is not clear (to me) why that ports ends up with corrupted
shared libs, reverting those changes fixes the issue and should allow us
to close p2k17 more smoothly.

Discussed with a bunch, ok ajacoutot@ guenther@

If the init_routine used with pthread_once() is canceled, then we need to
unlock the mutex inside the pthread_once_t. Push a cleanup wrapper for that.

Diff from Scott Cheloha <scottcheloha@gmail.com>
ok mpi@

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@