When arpwatch is terminated cleanly by a signal (INT/TERM/HUP) handler, the
exit code should be zero for success instead of nonzero for failure.

diff -Naur arpwatch-3.1-original/arpwatch.c arpwatch-3.1/arpwatch.c
--- arpwatch-3.1-original/arpwatch.c	2019-11-30 13:35:23.000000000 -0500
+++ arpwatch-3.1/arpwatch.c	2020-11-08 12:55:51.429575973 -0500
@@ -892,7 +892,7 @@
 
 	lg(LOG_DEBUG, "exiting");
 	checkpoint(0);
-	exit(1);
+	exit(0);
 }
 
 RETSIGTYPE