Various improvements and bug fixes

[daimrod]

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