Update mailbox when opening it
Close connections other than the one we chose

We can now just send :stop to the connection, so this is easy to do.
Let's make sure we do this after we've remembered the chosen connection,
to avoid being disrupted by the sentinel.
Add bic-stop, to stop an account FSM

I don't expect it to be commonly used, but I feel it's important to have
there, to be able to get rid of something that lives inside your Emacs.