Increase playerlimit

[elexis]

Currently the engine has an 8 player limit, but it could be entertaining to play with even more.

This is a non-trivial change as probably many data structures in C++ components rely on that limit and maps being designed for at most 8 players.

The performance shouldn't be affected significantly if the mapsize stays the same and the population limit is reduced proportionally. As of r17881 the network code already supports up to 24 clients without a noticeable performance impact.