Grid room level style (K-Mod)

Standard rooms-and-corridors levels have a small chance to be created in
the new grid level style: rooms of equal size are arranged in a grid 4-6
rooms wide and 2-3 high.  Corridors only join rooms that are directly
horizontally or vertically adjacent.
Raise chance of special rooms-and-corridors level style from 10% to 20%

This is the same chance that K-Mod has, so we may as well adopt it.