Make sure we do notify_remote_exit(ssh->frontend) _before_

Make sure we do notify_remote_exit(ssh->frontend) _before_
connection_fatal(), since the latter is entitled to destroy the
backend so `ssh' may no longer be valid once it returns.

For the Unix port, switch exit(0) to gtk_main_quit() in
notify_remote_exit(), so that we don't exit before the subsequent
connection_fatal()!

[originally from svn r5445]