shoes-off.el: Allow per bouncer port configuration

* shoes-off.el (shoes-off-server-port shoes-off-start): For more consistency,
`shoes-off-server-port' is now an integer.
(shoes-off-config shoes-off-start-session): Permit to configure the port
per bouncer.
shoes-off.el: Add an option to customize the host

* shoes-off.el (shoes-off-server-host): Default host for all bouncers
(possibly customizable per bouncer in `shoes-off-config').
(shoes-off-config shoes-off-start-session): Per bouncer configuration of
the host.
(shoes-off/make-server shoes-off-start/host-history shoes-off-start): Add
the host as a new argument.

This changes make it possible to customize the host in the same way we can
change the port.
shoes-off.el (shoes-off-stop): Improve shoes-off-stop

- check whether a shoes-off instance is running before killing it;
- set the variable storing the process to nil once it has been deleted;
- print a message to tell what happened;
shoes-off.el (shoes-off/get-session): Handle connection name bug.

The username stored is already username@service so you can't concat the
service a second time.