Dungeongen: Add and improve parameters

Dungeongen: Add and improve parameters

Add:
Bool for 'only_in_ground'.
Min and max corridor length.
Min and max room size with X, Y, Z components.
Min and max large room size with X, Y, Z components.

'only_in_ground = false' allows core mapgens to create structures
in air and water using dungeongen.
Corridor length parameters replace a fixed random range.
Room size parameters replace the former system where one parameter
'roomsize' was added to fixed random ranges.

All parameters are set for no change to current dungeon behaviour.

Remove some now-redundant and long-unused code.