Unsatisfied clients shouldn't spam logs; rate-limit them.

There's a constant UNSATISFIED_EXPIRY, and when a previously unsatisfied
client is idle for this many seconds, its entry times out and dhcpd emits
a syslog message saying how many questions did it ask in a time interval.

Upon exit the daemon writes out all entries that haven't yet expired.

Don't log these with DHCPRELEASE, just say "successful or not".  We might
have forgotten about the lease she's releasing.

Signed-off-by: Martin Pelikan <pelikan@storkhole.cz>