Implement freezing on Windows handle sockets.

Implement freezing on Windows handle sockets.

That's been a FIXME in the code for ages, because it's difficult to
get winhandl.c to stop an already-started read from a handle (since
the read is a blocking system call running in a separate thread). But
I now realise it isn't absolutely necessary to do so - you can just
buffer one lot of data from winhandl and _then_ tell it to stop.

git-svn-id: http://svn.tartarus.org/sgt/putty@10067 cda61777-01e9-0310-a592-d414129be87e