suppress EINTR in sem_wait and sem_timedwait

per POSIX, the EINTR condition is an optional error for these
functions, not a mandatory one. since old kernels (pre-2.6.22) failed
to honor SA_RESTART for the futex syscall, it's dangerous to trust
EINTR from the kernel. thankfully POSIX offers an easy way out.

src/thread/sem_timedwait.c

@@ -21,7 +21,7 @@ int sem_timedwait(sem_t *restrict sem, const struct timespec *restrict at)
 		a_cas(sem->__val, 0, -1);
 		r = __timedwait(sem->__val, -1, CLOCK_REALTIME, at, cleanup, sem->__val+1, sem->__val[2]);
 		a_dec(sem->__val+1);
-		if (r) {
+		if (r && r != EINTR) {
 			errno = r;
 			return -1;
 		}