Make sure we delete the PID file when we exit.

Obtained from:	OpenBSD rev 1.3 (this semi-security fix was hidden as a
                                 lease time fix)

contrib/isc-dhcp/client/dhclient.c

@@ -307,6 +307,8 @@ static void usage (appname)
 
 void cleanup ()
 {
+	/* Make sure the pidfile is gone. */
+	unlink (path_dhclient_pid);
 }
 
 /* Individual States: