Almost _all_ of the final connection-layer loop, in both SSH1 and

SSH2, is now handled by the packet dispatch table. Dispatch table
entries are enabled as soon as possible, so that if anyone tries to
(for example) start using a forwarded port before the main shell
session setup has finished, things should work sensibly.

The SSH code is now a hybrid of coroutine-based sequential logic and
table-driven event dispatch, each where it makes the most sense. I'm
rather pleased with it.

Should fix: ext-data-at-start, portfwd-at-start.

git-svn-id: svn://svn.tartarus.org/sgt/putty@4909 cda61777-01e9-0310-a592-d414129be87e