netsniff-ng: Exit gracefully for all possible termination signals

Handle all termination signals that we're allowed to handle (SIGKILL
can't be handled) in order to exit gracefully in any regular termination
case. Without this fix, pcap files written by netsniff-ng might be
corrupted.

Reported-by: Mike Westmacott <mikewestmacott@googlemail.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>