Turn hardcoded gadget defs into presets

[sprunk]

Some things don't fit in a unit def customparam and have to be defined in a dedicated config file, e.g. the definition of dirtbag mound:

Zero-K/LuaRules/Configs/unit_terraform_defs.lua

Lines 2 to 8 in 0b2bfbb

	[UnitDefNames["shieldscout"].id] = {
	shrakaPyramidDiff = 140,
	posX =
	{-8, 0, 8,
	-16,-8, 0, 8, 16,
	-24,-16,-8, 0, 8, 16, 24,
	-32,-24,-16,-8, 0, 8, 16, 24, 32,

But it's defined per-unitdef, so if a modder copies Dirtbag then it won't have the feature. Similar things happen with drones and perhaps others (part of the task is to find the others).

Change this so it's a named preset, e.g. small_mound = { ... } in the config and then Dirtbag gets death_terraform = "small_mound" customparam. Then a ModdedDirtbag (or generally any unit) can also get the customparam to get the behaviour.