If a TCP connection has been idle for one retransmit timeout or more

If a TCP connection has been idle for one retransmit timeout or more
it must reset its congestion window back to the initial window.

RFC3390 has increased the initial window from 1 segment to up to
4 segments.

The initial window increase of RFC3390 wasn't reflected into the
restart window which remained at its original defaults of 4 segments
for local and 1 segment for all other connections.  Both values are
controllable through sysctl net.inet.tcp.local_slowstart_flightsize
and net.inet.tcp.slowstart_flightsize.

The increase helps TCP's slow start algorithm to open up the congestion
window much faster.

Reviewed by:	lstewart
MFC after:	1 week