Generalise SSH_MSG_CHANNEL_{SUCCESS,FAILURE} handling.

Generalise SSH_MSG_CHANNEL_{SUCCESS,FAILURE} handling.

Now each channel has a queue of arbitrary handlers for those messages,
with anything that sends a CHANNEL_REQUEST with want_reply true pushing
a new entry onto the queue, and a shared handler that dispatches
responses appropriately.

Currently, this is only used for winadj@putty.projects.tartarus.org, but
extending it to cover the initial requests as well shouldn't be too
painful.

git-svn-id: http://svn.tartarus.org/sgt/putty@9615 cda61777-01e9-0310-a592-d414129be87e