Fix not being able to join to Dwarf Fortresses that were "captured".

"captured" as in not launched by Dfterm3; something the user started themselves
and wrote 'start-dfterm3' to the Dfhack console.
Fix small hlint warning.
Keep track of who is in a game and send the information to web interface.

This is enough to implement player listing in the web interface. However, the
web interface implementation is not there yet. Maybe in the next commit?