Another tweak to EOF policy: invent an outgoing EOF on receipt of an

Another tweak to EOF policy: invent an outgoing EOF on receipt of an
incoming CHANNEL_CLOSE, if it's the main session channel. The idea is
that invocations such as 'plink -T hostname sh' (running a shell
without a remote pty) can be exited by typing 'exit' to the remote
shell, without plink blocking forever waiting for outgoing EOF.

I think it would be better to do the same for all other channel types
too, but that would need an extra API call which I haven't
implemented yet.

[originally from svn r9283]