Various improvements and bug fixes

In chronological order:
- Support the :encryption keyword. It was allowed in the defcustom but wasn't allowed by shoes-off-start-new-session. It is now both allowed and used.

- Allow the user the customize the port used by the bouncer directly in shoes-off-config

- Do the same thing but for the host. This way we can now restrain the bouncer to listen on localhost only.

- Minor improvement of shoes-off-stop

- Fixed a regression in shoes-off/get-session