Re-abstract out antimagic susceptibility.

I'm not certain we want to keep the check as is_actual_spellcaster() for
monsters, and if we decide to change, this will help do so. Either way
it being a method of actor saves some lines.
oops
Simplify.
Restore old anti-magic behavior.

This is accomplished by splitting MON_SPELL_INNATE into _NATURAL, _MAGICAL,
and _DEMONIC. The last two are affected by antimagic; they have no
differences for now, in fact, but it seems like a good idea to future-proof
anyway.
Fix some mistakes I made with flags in mon-cast.cc.