Add external cameras

F7 key by default, toggling between first person, rear and front. Only
works for the standard minetest_game player model. Changes the protocol
version.

This is based on BlockMen's work as per minetest PR#1114, with lots of
merging to bring it up to date, and a bunch of extra fixes.

This is a combination of 10 commits.

Conflicts:
	src/camera.cpp
	src/game.cpp
	src/hud.cpp
	src/client.cpp
	src/clientserver.h
	src/server.cpp
	src/server.h
	minetest.conf.example