Added possibility to reserve maps

If you set net_gameServerName you can now reserve maps in the maps.xml.
There you have to add the servername - property to the <map> tag.
Then the map will only be activated by that server.

Also changed the activate sequence that the account server now tells the game
server what maps to activate (previously the server requested all maps and the
account server said yes or no).

TODO: Fix general inter server map switching.