Don't leave cargo stuck on land unit killed by che

Don't leave cargo stuck on land unit killed by che

When che destroy a land unit, any embarked units remain stuck on their
now dead carrier.  Closely related to and same impact as the bug fixed
in commit 8ccad0d7.  Broken since Chainsaw 3 added land units.

The obvious fix would be to match what normally happens when a carrier
gets destroyed: destroy the cargo.  Requires recursion.  To keep
things as simple as possible, destroy plane cargo, but unload land
unit cargo.  That way, the only cargo of cargo to visit are nukes on
planes.

Unloading the land units creates another problem, which will be
addressed in the next commit.