[master] Add define to disable gentle shutodwn by default

Add a define to disable the gentle shutdown feature by default.
The feature has some effects (recover-wait in failvoer pairs)
and release in clients that are always desired.  We plan to
revisit this in order to make it easier to use but with the
define it can be enabled if necessary.

server/dhcpd.c

@@ -787,9 +787,12 @@ main(int argc, char **argv) {
 	omapi_set_int_value ((omapi_object_t *)dhcp_control_object,
 			     (omapi_object_t *)0, "state", server_running);
 
+#if defined(ENABLE_GENTLE_SHUTDOWN)
+	/* no signal handlers until we deal with the side effects */
         /* install signal handlers */
 	signal(SIGINT, dhcp_signal_handler);   /* control-c */
 	signal(SIGTERM, dhcp_signal_handler);  /* kill */
+#endif
 
 	/* Log that we are about to start working */
 	log_info("Server starting service.");