Cleaned up common/spdat.cpp

There was a lot of stuff that I found that the level of readability.
I cleaned up a lot of the stuff, no functionality should be changed
unless where noted below.

* IsPartialDeathSaveSpell is now generic and no longer uses a hardcoded spell id
* IsFullDeathSaveSpell is now generic and no longer uses a hardcoded spell id

If these functions were actually used in the spell effects it would probably
fix issues with newer spells (which there are on live)