Factor out all local SSH disconnections into a new function ssh_disconnect(),

Factor out all local SSH disconnections into a new function ssh_disconnect(),
and add the ability to treat a local disconnection as "unclean" -- notably, if
we can't agree any authentication methods to even try; someone was complaining
that the PuTTY window by default just disappears for no apparent reason in this
circumstance.

Also, use appropriate disconnect codes for those SSH2_MSG_DISCONNECT messages
that we do send.

I don't think I've seriously broken any user-visible behaviour, but the way
that connection-close distinctions are transmitted to the front-end is shaky
(or so it seems to me), so there may be non-ideal changes on some platforms.

[originally from svn r5824]