core: Ensure use of Signals enum
core: Ensure use of Signals enum

_install_signal_handler() is only meant to be called with values that
are legal signal.Signals entries.  If it's called with an unexpected
value, it now fails in an obvious way.

MurphyMc/pox,MurphyMc/pox,MurphyMc/pox,MurphyMc/pox,MurphyMc/pox