Support for big games checked in

[Stan Shebs]

Well, it was bound to happen sooner or later - Jose' Manuel Sebrosa
wrote me about a problem with saving where the unit ids had
overflowed(!), went negative, and proceeded to cause some havoc.

After I picked my jaw up off the floor, I found out that he was
playing with lots of AIs at large advantages, and got the overflow
after about 200 turns.  (He didn't actually have 32,000 units on
the board at once, it's just that that many units had been
created and destroyed during the game.)

While it's conceivable to try to recycle unit ids, that entails
more code; so instead I bumped the unit id from 16 to 32 bits,
and grew other data structures (such task arg lists) to match.
Although this grows Xconq's memory footprint, the effect is small
compared to some other changes.

Testing showed everything still working as expected, so I checked
it all in.

Stan