Better handling of outstanding CHANNEL_REQUESTS on channel destruction.

Part the first: make sure that all structures describing channel
requests are freed when the SSH connection is freed.  This involves
adding a means to ask a response handler to free any memory it holds.

Part the second: in ssh_channel_try_eof(), call
ssh2_channel_check_close() rather than emitting an SSH_MSG_CHANNEL_EOF
directly.  This avoids the possibility of closing the channel while a
CHANNEL_REQUEST is outstanding.

Also add some assertions that helped with tracking down the latter
problem.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9623 cda61777-01e9-0310-a592-d414129be87e