Servatrice password authentication

Implements #290, and adds a lot of nice comments to servatrice's example configuration file.
Depends on #353 
I only daw the code that makes servatrice abort its startup if regonly=1 but no db has been configured, but i'm not been able to identify the code that actually avoids unregistered users from logging in. Since it's the first time i'm digging in servatrice code i'm probably missing out something obvious here...