exit with EXIT_SUCCESS (aka 0) when stopped by signal

Rationale: When mcelog is running as a daemon, sending a SIGQUIT/SIGTERM/SIGINT is the only way to gracefully stop mcelog. This is especially useful in combination with systemd since systemd uses the exit value to detect if something got wrong.
Merge pull request #12 from genodeftest/patch-1

exit with EXIT_SUCCESS (aka 0) when stopped by signal