Option to log proxy setup diagnostics to the terminal.

Option to log proxy setup diagnostics to the terminal.

It has three settings: on, off, and 'only until session starts'. The
idea of the last one is that if you use something like 'ssh -v' as
your proxy command, you probably wanted to see the initial SSH
connection-setup messages while you were waiting to see if the
connection would be set up successfully at all, but probably _didn't_
want a slew of diagnostics from rekeys disrupting your terminal in
mid-emacs once the session had got properly under way.

Default is off, to avoid startling people used to the old behaviour. I
wonder if I should have set it more aggressively, though.