Revision 284941

Avoid a situation where we do not set persist timer after a zero window
condition.
If you send a 0-length packet, but there is data is the socket buffer, and
neither the rexmt or persist timer is already set, then activate the persist
timer.

PR:			192599
Differential Revision:	D2946
Submitted by:		jlott at averesystems dot com
Reviewed by:		jhb, jch, gnn, hiren
Tested by:		jlott at averesystems dot com, jch
MFC after:		2 weeks