I think this should fix various problems with queued incoming data not being

processed and incoming data being processed out of order, which I suspect is
the cause of `ssh1-fwd-trouble' as noted by Gevan Dutton. I'm not able to
test the failure case, but it doesn't seem to have obviously broken anything
in the cases I have tested, anyway.

git-svn-id: svn://svn.tartarus.org/sgt/putty@6221 cda61777-01e9-0310-a592-d414129be87e