fix #H184 - rndmonnum plan B

fix #H184 - rndmonnum plan B

     From a bug report, the fallback selection criteria
(used when everything is extinct?) in rndmonnum() was excluding hell-only
monsters when outside of Gehennom, but failed to exclude never-in-hell ones
when inside.  [Some of the never-in-hell monsters are Angels, but the rest
are all cold based creatures.  That must date to when fire resistance was
required for the hero, which is no longer the case.  Should those cold
monsters retain their never-in-hell setting?]

     This also fixes a latent copy/paste bug in the unused mons[] definition
of Cerberus (it was the only unique monster which failed to specify G_NOGEN).