Dungeons: Clean up parameters, improve structure variety (#8918)

Dungeons: Clean up parameters, improve structure variety (#8918)

While preserving the general character of dungeon structure.
Slightly increase the range of standard room horizontal size, while
preserving the average horizontal size.
Return to classic maximum large room size of 16x16x16.
Make 1 in 4 dungeons have a 1 in 8 chance for each room being 'large',
making multiple large rooms possible for the first time.
Make 1 in 8 dungeons allow diagonal corridors, to make these a little
more common.
Make corridor width vary from 1 to 2, but forced to 2 if diagonal
corridors are allowed, to make them passable.
Add some comments.