Fix sign compare error reported by gcc with -Wextra

Signed-off-by: Christophe Vu-Brugier <cvubrugier@fastmail.fm>
Handle all signals with signalfd

This prevents unhandled signals to be caught by the default signal
handler whose action is to terminate the process.

This patch fixes issue #13.

Signed-off-by: Christophe Vu-Brugier <cvubrugier@fastmail.fm>