spurious mail daemons (trunk only)

spurious mail daemons (trunk only)

     From the newsgroup:  when the special level loader is creating
random '&' class monsters, it can produce mail daemons.  The thread
reports seeing some on the levels holding the Wizard of Yendor's Tower
and they vanish as soon as it's their turn to move.  I didn't reproduce
it, but create_monster() is overriding the NOGEN flag when it calls
mkclass() so getting mail daemons is feasible.  This fixes that by
treating them similar to the human/elf/orc/giant placeholder monsters
that are excluded from random generation.  More or less.