Let resistant monsters ability to enter clouds.

Also adds some previously-un-handled cases for when a non-immune but tough
monster enters a cloud that won't hurt it too bad. The magic numbers
in those cases are probably wrong but I don't know what I'm doing with
those.
Cloud updates now with sane magic number choices
Simplify more _mons_avoids_cloud() code
Pull expected cloud damage into data
Move cloud names into the new cloud struct
Make very dumb creatures not fear rain

Specifically, not fear drowning in a shallow pool.

They are very stupid, these monsters.