o Move setting/resetting logic of syncache timer from macro

SYNCACHE_TIMEOUT to new function syncache_timeout().

o Fix inverted timeout callout engagement logic to actually

enable the timer for the bucket row.  Before SYN|ACK was
not retransmitted.

o Simplify SYN|ACK retransmit timeout backoff calculation.
o Improve logging of retransmit and timeout events.
o Reset timeout when duplicate SYN arrives.
o Add comments.
o Rearrange SYN cookie statistics counting.

Bug found by: silby
Submitted by: silby (different version)
Approved by: re (rwatson)