Switch the license to be LGPLv2/later, with small parts still remaining as GPLv2/later, by agreement of major contributors

- 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

--HG--
rename : src/licensecomment.txt => licensecomment.txt