Demon lairs: Demogorgon, Geryon, Dispater, Yeenoghu (GruntHack)

This adds to Gehennom demon lairs for (in descending order of likely
depth): Yeenoghu, Dispater, Geryon and Demogorgon.

Demogorgon definitely earns his title of hardest enemy in NetHack.
DynaHack doesn't let the player get as overpowered as in GruntHack, so
now that Demogorgon is guaranteed, there needs to be better ways to deal
with him.
Exclude x == 0 from level flipping

This fixes the long-standing UnNetHack level flipping bug, where a
horizontally flipped mines-style level would be one space too close on
the left side (causing ugly wallification) and have an extra space on
the right side (causing such levels to appear misaligned on the screen).

Fixes #4 on GitHub.
Give back some gold to Gehennom

The removal of mazes from Gehennom also accidentally took away the 7 to
14 random gold piles added by the maze code too.  This commit brings
back 1 to 7, which should be enough to bribe a few demon princes once
UnNetHack's bribery cost has been scaled back a bit.
Reduce demon bribery requirement; give more leeway

To guarantee a demon prince will be successfully bribed, you'll need
2000 to 6000 gold (2000 to 3000 for characters whose alignment matches
the demon prince's), but they may demand more if you carry more than
that in open inventory.

You can now offer down to 80% of their bribery demand too, with success
scaling down the less you offer.
Allow Demogorgon to be bribed

Also make him lawful like the other bribable demon princes.  Given how
tough Demogorgon is in a fight, this is the recommended way of dealing
with him.  Like other demon princes, don't wield Excalibur while
entering his level or approaching him or you won't be able to bribe him
at all.
Remove unused global var goldseen from mklev.c

Apparently it hasn't been used for anything since NetHack 3.0.0;
something to do with mimics and random gold piles.
Reduce max difficulty of initial monsters on levels

Instead of producing monsters with difficulty to dungeon depth 1-to-1,
do that for the first 10 levels and then 1-to-2 thereafter.

Considering an XL 10 character at the Castle (depth 25-30) the
difficulty becomes:

 * NetHack: 17 to 20
 * Nitro/NH4/DynaHack (before): 25 to 30
 * DynaHack (now): 17 to 20

Considering an XL 18 character in Moloch's Sanctum (depth 50):

 * NetHack: 34
 * Nitro/NH4/DynaHack (before): 50
 * DynaHack (now): 30