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)
Fix database scheme comments that got clobbered on nullptr conversion
Fixed non-attacking related movement while mobs are snared

On live a nec/shd normal splitting involved snaring the mob you want
and FDing after they moved enough. The non-snared mobs would return home
and the snared mob would just sit there until it was no longer snared.
There is even a nice flash video! http://www.dzert.com/downloads/eq/fd02.swf
This might need some tweaking, but I'm not sure of any instances where it would
break things...
Revert "Fixed non-attacking related movement while mobs are snared"

Did some testing, this isn't exactly the case on live.

This reverts commit ed6c46f7ffa2a8155e0f22cc52730762df7aee35.
Fix walk speed calculation

Walk speed should be calculated as a movement speed reduction that
stacks with snares so it's possible to snare a mob FD and have it
not be able to walk back.