Experimental fix for `psftp-hang'. (Experimental only in that I haven't thought

about it very hard; it's a plausible fix for the observed cause of the extreme
CPU usage, being that we were asking to be notified of Windows messages and
then not dealing with them, plausibly leading to a loop. Works for me,
anyway.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@7098 cda61777-01e9-0310-a592-d414129be87e