Fix multiple bugs in freeze/thaw of Windows handle-sockets.

Fix multiple bugs in freeze/thaw of Windows handle-sockets.

Firstly, I had asserted that data would never arrive on a handle
socket in state FREEZING, which is just an error, because FREEZING is
precisely the state of not being quite frozen _yet_ because one last
read is still expected to arrive from the winhandl.c reading subthread
which it's too late to cancel. I meant to assert that it wasn't
FROZEN.

Secondly, when the handle socket was in state FREEZING, I failed to
actually _set_ it to FROZEN.

And thirdly, when the handle socket starts thawing again (i.e. there's
now outgoing buffer space so we can start sending our backlogged
data), I forgot to ever call bufchain_consume, so that the same block
of data would get sent repeatedly.

I can only assume that nothing I've ever done has actually exercised
this code!