Prevent polymorphing into force-depth monsters.
Allow monsters to be placed on any monster-walkable features.
Add a safety check to at least prevent crashes.
Add a warning message.
Change names of some constants.
Use constants in monster_desc().
Change some monster_desc flags.
Merge pull request #268 from bensemmler/polymorph

Try to prevent polymorph crashes, and add an in-game message when we do so. Addresses #1765.
Merge pull request #269 from bensemmler/monster-desc

Work towards fixing #1519, monster descriptions being a bit off.
Fix for room_template.txt provided by Nomad
Start mushrooms appearing later in the dungeon, spread out a bit more
Double the change of finding ,Vigor, drop it to start lower
Fix up be0660df, which tried sometimes to place new monsters where there was already a monster.