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.