Reimplement authentication handler in Lua; now we have 1) infinite privilege names, 2) minetest.register_authentication_handler()

Defaults modified from original: alpha=200, key=F10

- Node placement code moved to Lua
- Mineral system removed (added default:stone_with_coal and default:stone_with_iron).
- MapBlock and MapNode serialization updated.
- Mapgen: Frequency of iron increased.
- node_dig callback and related changes.

The huge item definition and item namespace unification patch (itemdef), see http://c55.me/minetest/wiki/doku.php?id=changes:itemdef

Irrlicht 1.8 compat); also find dungeon master's fireball texture
again; add /spawnentity command

inventorycube: use all three specified textures; also moved mesh creation / modification functions to mesh.cpp; in lua, inventorycube is now called minetest.inventorycube