Restore .Xr to pthread_sigmask

Pointed out by naddy@

Move sigwait(3) from libpthread to libc

POSIX wants it in libc, that's where the function can be found on other
systems. Reported by naddy@, input from naddy@ and guenther@.
"looks ok" guenther@, ok deraadt@

Note: riding the libc/libpthread major cranks earlier today.

Move sigwait(3) from libpthread to libc

POSIX wants it in libc, that's where the function can be found on other
systems. Reported by naddy@, input from naddy@ and guenther@.
"looks ok" guenther@, ok deraadt@

Note: riding the libc/libpthread major cranks earlier today.