Fix warnings and other misc. minor changes

src/emerge.h

src/mapgen.cpp

src/mapgen_v5.cpp

src/mapgen_v6.cpp

src/mapgen_v6.h

src/mapgen_v7.cpp

src/mg_biome.cpp

src/mg_biome.h

src/mg_ore.cpp

src/mg_ore.h

src/script/lua_api/l_mapgen.cpp