Move win32 early signal handling setup to os_setup_signal_handling()

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>