In the signal handler, instead of restoring the configuration of the
terminal and then raising the signal (SIGINT), a better practice is to
just stop the main loop, so that the main function can do the cleanup
and exit normally (as with the 'q' key).

Also register this signal handler for SIGTERM (sent by kill).

Signed-off-by: Cédric Marie <cedric@hjuvi.fr.eu.org>