Fix C343-415: Mail daemons can be created when populating special levels with demons

Special level generation in NetHack and UnNetHack mask out G_NOGEN when
randomly creating monsters from a class, leading to mail daemons on
special levels that ask for '&' and chromatic dragons when they ask for
'D'.

This is inspired by a fix in GruntHack (also ported to NetHack4), but
replaces the hard-coding of Kops and eels with logic that checks that
all monsters of a class are flagged G_NOGEN before making an exception.

This replaces commit 0b268abae3 (respect G_NOGEN when making mon by
class on splevs).
CS1542: Remove gen freq from chromatic dragons

Generation frequency is pointless on a monster that is never randomly
generated.  In UnNetHack, this fixes randomly appearing chromatic
dragons, but this dev branch already fixes C343-415, so it isn't an
issue here.
CS1290: Re-add scrolls of earth to Sokoban

They give players more options to recover from mistakes or creatively
solve Sokoban, so I never agreed with their removal, and I'm glad
they're back.
Always give Cthulhu the Amulet of Yendor and a potion of full healing

For some reason I un-hardcoded these when I ported UnNetHack r644, even
though UnNetHack itself didn't.  For the sake of unity I'm making them
the same again.

This change will only be noticed if Cthulhu isn't generated in the
Sanctum, which should never happen in a normal game.