Cut out snowflake mob_can_equip code.

Now it calls mob/proc/can_equip(obj/item/I, slot, disable_warning = 0) instead.

Good-riddance, nasty ugly if(istype(...))!


Future task: update corgi/xeno/whatever code to use this proc, now that it's nice and clean.
Added can_equip proc for polymorphism.
Made human-specific can_equip proc.

This code is still a mess, but it probably can't be helped short of rewriting inventory code with standardized slot numbers and such.
Quick fix on can_equip.

I realized I forgot about the warning messages.
Added monkey/can_equip proc.

See, isn't that much nicer?
Trying to fix compilation error.

Changed "M as mob" in arguments list to "mob/M"; I think the former made it look for proc/can_equip() instead of mob/proc/can_equip().


Also, it no longer returns 0 for a null slot parameter.
Merge pull request #3424 from Nnelg/Polymorphic-can_equip

Polymorphic can_equip