Several time related improvements:

- set initial delay to 0 to speed up client start
- added 'initial-delay' parameter to possibly revert to old behavior
- better handling of very short (1 or 2s) leases
- client lease records are recorded at most once every 15 seconds
- ICMP ping-check is now timed more precisely
- Servers that don't offer lease-time are now black-listed
[ISC-Bugs #19660]

server/dhcp.c

@@ -2836,8 +2836,14 @@ void ack_lease (packet, lease, offer, when, msg, ms_nulltp, hp)
 		log_debug ("Ping timeout: %ld", (long)ping_timeout);
 #endif
 
-		tv . tv_sec = cur_time + ping_timeout;
-		tv . tv_usec = 0;
+		/*
+		 * Set a timeout for 'ping-timeout' seconds from NOW, including
+		 * current microseconds.  As ping-timeout defaults to 1, the
+		 * exclusion of current microseconds causes a value somewhere
+		 * /between/ zero and one.
+		 */
+		tv.tv_sec = cur_tv.tv_sec + ping_timeout;
+		tv.tv_usec = cur_tv.tv_usec;
 		add_timeout (&tv, lease_ping_timeout, lease,
 			     (tvref_t)lease_reference,
 			     (tvunref_t)lease_dereference);