Display mp::wait (temporary dummy) in between mp::scenarios.

The screen is displayed only if '--campaign-screens' command line
option was specified. The same now goes for mp::connect screen for the
game's host.

Server has been modified to not immediately start the game after
'store_next_scenario', but to wait for explicit 'start_game'.

Minor changes to parameter passing inside multiplayer.?pp have been
made as well.
Move the init of leaders and genders list from mp::connect to mp::ui.
Unify leaders and genders set up between mp::connect and mp::wait.

The relevant functions from mp::connect were moved to mp::ui to enable
them to use by mp::wait as well. This is not the most elegant solution,
but in the future mp::connect and mp::wait should be merged into one,
and those functions could be moved back to where they were.

leader_list.?pp has been removed since it is no longer used.
Improve factions, leaders & genders related code.
Revert to a default login in case of an EMPTY_WRAPPED_STRING.
Refactor users related code (mp::connect).