fix SupplyLow flag on give/take

[Jim Kingdon]

> Your patch is correct and worth installing (thanks!).

Done.

> There should probably be versions of past_halfway_point that don't
> test for being in a transport, but that's where the cases start to
> multiply! (owww... :-) )

I'd say stick with the status quo (or, conceivably, take out the
transport check in *all* cases).  If you really wanted SupplyLow to be
more than a rough heuristic kind of thing, the player would need to be
able to set how conservative they are (e.g. hitting a submarine on the
way back may require you to go out of your way and no longer be in
range), where they want to resupply a particular unit from, and the
like.  Not worth the complexity for this particular feature, as far as
I can see.