To avoid misunderstanding, keep variables for tcp keepalive in

To avoid misunderstanding, keep variables for tcp keepalive in
milliseconds, which is the same unit of tcp_now().  However, keep the
unit of sysctl variables in seconds and convert their unit in
tcp_sysctl().  Additionally revert TCPTV_SRTTDFLT back to 3 seconds,
which was mistakenly changed to 1.5 seconds by tcp_timer.h 1.19.

ok claudio