Tnt: Various optimisations

Pass nodename to tnt.burn function where possible to reduce
use of 'get_node'.
Change 'ipairs' to 'pairs'.
Use 'nodeupdate_single(pos)' instead of 'nodeupdate(pos)' to
avoid every node triggering recursion, the loop itself takes
the place of recursion and works upwards through horizontal
planes as required.

game_api.txt

@@ -290,9 +290,9 @@ TNT API
 * `position` The center of explosion.
 * `definition` The TNT definion as passed to `tnt.register`
 
-`tnt.burn(position)`
+`tnt.burn(position, [nodename])`
 
-^ Ignite TNT at position
+^ Ignite TNT at position, nodename isn't required unless already known.
 
 
 To make dropping items from node inventories easier, you can use the