rename team::is_human() -> is_local_human

and is_ai -> is_local_ai
remove "network_ai" and "network" controllers

an instead have a filed "is_networked" in [side]. The plan is to remove
"is_networked" and to replace it with a 'controller_client_id ==
local_controller_client_id' check (names like controller_client_id might
change).

This might break existent wml that uses side [controller] to know
whether a side is networked, the plan is to add a lua fucntion
wesnoth. is_networked(side). These change in wml are not side effects
they are inteded. Becasue this makes it easier to play Sp scenario that
doesn't think about "network_ai" and "network" controllers in mp, and it
disallows stuff like [modify_side] controller="network" which always
breaks mpsync.