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