Fix signal handling when io-thread is disabled

Fix signal handling when io-thread is disabled

Changes since v1:
- take pthread_sigmask() out of the ifdef as it is now common
to both parts.

This fix effectively blocks, in the main thread, the signals handled
by signalfd or the compatibility signal thread.

This way, such signals are received synchronously in the main thread
through sigfd_handler() instead of triggering the signal handler
directly, asynchronously.

Signed-off-by: Alexandre Raymond <cerbere@gmail.com>
Acked-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>