Replace GetQueueStatus with PeekMessage(PM_NOREMOVE).

A couple of users report that my recent reworking of the Windows
top-level message loop has led to messages occasionally being lost,
and MsgWaitForMultipleObjects blocking when it ought to have been
called with a zero timeout. I haven't been able to reproduce this
myself, but according to one reporter, PeekMessage(PM_NOREMOVE) is
effective at checking for a non-empty message queue in a way that
GetQueueStatus is not. Switch to using that instead. Thanks to Eric
Flumerfelt for debugging and testing help.


git-svn-id: svn://svn.tartarus.org/sgt/putty@10057 cda61777-01e9-0310-a592-d414129be87e