signals: implement sigwait

MongoDB will call sigwait and expect the process to actually stop, so we can't
stub them. Also, some of its features may depend on the fact that the program
will resume after a wait if a signal actually reaches it, (for cleanup and
whatever).

So let's come up with an actual implementation.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>