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

Add minetest.register_on_joinplayer and minetest.register_on_leaveplayer,
make adding new callbacks to scriptapi.cpp easier by adding
scriptapi_run_callbacks, also fix a minor bug with PlayerSAO <->
singleplayer mode interaction

- 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

Make global names in default mod use good naming convention and move backwards-compatibility definitions to mods/legacy