move AI init from NETMSG_STARTPLAYING to CGame::LoadGame

This change will affect Native AI initialization (and Lua's also). ZK tests were good (CAI, Chickens, Shard, CppTestAI, own LocalTestAI with sendStartPosition at EVENT_INIT). It is possible to make separate initialization for Lua and native AIs with SkirmishAIData::isLuaAI flag if something is going to go wrong for Lua.

1 limitation for native AIs: they should use
this.callback.sendStartPosition(false, position);
instead of
this.callback.sendStartPosition(true, position);
Somehow "true" will signal that the player (not AI only) is ready.