New Xconq unit type prop "encounter-result"

[Stan Shebs]

I've just committed a change to emulate the "goodie huts" of
Civ.  This happens with a new unit type property encounter-result,
that says what happens if any unit attempts to overrun a cell
containing a unit with a nonempty encounter-result.  Possible
results include: vanishing, changing to a type on your side,
or changing to a type on a fixed side.  The list is a weighted
list a la side-library, so you can fine-tune the odds, and you
can specify a notification string for players.

Right now this code is tied to "model 1" (Civ-type) combat, but
it wouldn't be too hard for an enterprising hacker to attach it
to any combat model.  It should be tied to overrun actions in
any case, because overrun is the first action type tried when
players click on a cell to move into.

Stan