Mainloop: handle SIGINT as well (and terminate)

This is needed if daemons are in the foreground, and get ctrl+c-ed by
the user. Also add unittests to make sure the correct signals terminate
the mainloop.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>