Mapgen: Various fixes and improvements

Lua_api.txt: Document 'minetest.registered_biomes'
Minimal: Remove 'mapgen_air' alias
Cavegen: Add fallback node for 'mapgen_ice'
Dungeongen: Add fallback node for 'mapgen_river_water_source'
Mgv5: Remove unnecessary '#include util/directiontables.h'
Add missing 'this->'s in makeChunk()
Mgv6: Edit empty line formatting
Remove leading spaces in makeChunk()
Add missing spaces after 'for' and 'if'
Mgv7: Edit empty line formatting