[xms] new gamemode: arena

[utharper]

On the map arenaplex_v2 (the default map for this mode), 2 players will spawn into a randomly selected arena. The other players watch them from the designated spectate areas, until one of them emerges victorious. The champion will then battle a randomly selected challenger, in a randomly selected arena, and so on until the fraglimit is reached.

In any other map the functionality is the same, except the 2 chosen players will spawn into the map as normal instead of any particular map coordinates. The other players will need to be forced into the spectator team.

First step is establishing the spawn coordinates for each arena in Arenaplex.