Refactor prototype REPL for SSH / login access

[oubiwann]

Tasks:

• Support ssh'ing into game shell
• Run each shell in its own LFE process with its own state
• Set user data in that shell process' state
• Maybe perform various user look-ups as part of login (e.g., cache role-based data) so that DB queries may be avoided if possible -- store these in the login state, too
• Add readline support
• Enable only one instance of a single character logging in at a time
• Ensure multiple logins with different users work as expected

Probably best to finish the TCP registration server first ... (see #3)