Midgame join leave fix

Mid-dungeon joiners should now reliably join a party, and mid-dungeon party leaders leaving should now not delete their entire party.

Also changes twopset so it splits control of bots evenly between two players, regardless of their positions in the party.

When a party is deleted, maxplayers is adjusted to account for that, and when the second-to-last party is deleted in PVP, the server is sent back to the lobby.

This merge request also includes a fix for a minor issue where a player changing names ends up changing skins and back again without having used the skin command.