Check the size of the Lua stack when unpickling.

On my box, I actually hit the jackpot when trying to load the #2986
game.
Merge remote-tracking branch 'laarmen/checkstack_unpickle'
Use the latest equipment API in CargoBody instanciation (Closes #2992)
Change the GetCommodityBasePriceAlteration interface

This might allow for better diagnosis of what's going on in the various
recent crashes. Or not. But at least we don't build a whole new table
every time this method gets called, which is a win to me. Plus, the code
is simpler.