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.

[originally from svn r10067]