Introduce os-posix.c and create os_setup_signal_handling()

Introcuce os-posix.c and move posix specific signal handling
there.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Acked-by: Juan Quintela <quintela@redhat.com>
Acked-by: Richard Henderson <rth@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>