No, I take that back: we _do_ have a mechanism for suppressing reads

No, I take that back: we _do_ have a mechanism for suppressing reads
from forwarding data sources which will be good enough to last until
we close the socket, in the form of the override_throttle() functions.
So this finishes up the work in r9283, by manufacturing outgoing EOF
in response to incoming CLOSE on all channel types.

git-svn-id: http://svn.tartarus.org/sgt/putty@9284 cda61777-01e9-0310-a592-d414129be87e