Refactor users related code (mp::connect).
Re-enable lobby to show reloaded game titles.
Remove obsolete "simple server" related code (mp::connect).

As a result of this, 'connected_user' struct was no longer useful and
is now removed. Connected users information are now being hold in
std::set<std::string>, because only their unique usernames matter
in mp::connect.
Minor clean-up of users related code (mp::connect).
Allow non-reloaded games to use "Reserved" option.