signal handling: support SA_RESETHAND

Add support for SA_RESETHAND signal handler flag, which means that the signal
handler is reset to the default one after handling the signal once.

I admit it's not a very useful feature (our default handler is powering off
the system...) but no reason not to support it.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>