[crossfire] destructor archetype?

[mwedel at sonic.net]

 Couple quick thoughts on this:

 Rather than using a flag to denote 'destructor' abilities, I think it
would be better to make it a subtype.

 In fact, in an ideal world, anything that looks for objects being dropped
on the space would have the same object type, but then different subtypes
based on what it does (duplicate, destroy, sacrifice, etc).

 In that way, the 'check if on space' logic becomes the same for all the
objects, which is probably a good thing and would simplify the code, and
documentatin becomes 'if subtype is ABC, it does this, using these
fields'.