polyself changes (trunk only)

polyself changes (trunk only)

     Several polymorph tweaks, most dealing with specifying form under
polymorph control or for wizard #polyself:
1) allow "were<critter>" and "human were<critter>" for your type of
   <critter> when you're inflicted with lycanthropy; now you'll toggle
   shape rather than be told "you cannot polymorph into that".
2) allow your own role; now you'll become a new man (or whatever race)
   rather than get "you can't".
3) allow "human" to force a new man (or whatever) regardless of race.
   No change for human characters, but elves, dwarves, and such can now
   use either their own race or "human".  (They never become humans.)
4) for wizard #polyself only, override the 20% chance of becoming a new
   man instead of taking on the selected form.  (This implicitly prevents
   the annoying "your new form isn't healthy enough to survive" death
   since your experience level won't drop below 1.)
5) remove a redundant drowning check in polyself(); it's already handled
   in polymon() and polyman(for newman()) via spoteffects().

     This also gets rid of an old use of 0 as not-a-valid-monster (not
responisble for any bugs though since giant ants aren't lycanthropes).