unify and overhaul timed futex waits

new features:

- FUTEX_WAIT_BITSET op will be used for timed waits if available. this
  saves a call to clock_gettime.

- error checking for the timespec struct is now inside __timedwait so
  it doesn't need to be duplicated everywhere. cond_timedwait still
  needs to duplicate it to avoid unlocking the mutex, though.

- pushing and popping the cancellation handler is delegated to
  __timedwait, and cancellable/non-cancellable waits are unified.

src/thread/pthread_mutex_timedlock.c

@@ -16,7 +16,7 @@ int pthread_mutex_timedlock(pthread_mutex_t *m, const struct timespec *at)
 		a_inc(&m->_m_waiters);
 		t = r | 0x80000000;
 		a_cas(&m->_m_lock, r, t);
-		r = __timedwait(&m->_m_lock, t, CLOCK_REALTIME, at, 0);
+		r = __timedwait(&m->_m_lock, t, CLOCK_REALTIME, at, 0, 0, 0);
 		a_dec(&m->_m_waiters);
 		if (r && r != EINTR) break;
 	}