damage: Shield embarked planes and land units from sector damage

damage: Shield embarked planes and land units from sector damage

Damage to a ship or land unit, say via pinpoint bombing, doesn't
damage loaded planes and land units.

Damage to a sector, say via strategic bombing, doesn't damage ships
there, but it does damage planes, even when loaded on a land unit (but
not when loaded on a ship), and land units, even when loaded on a land
unit or a ship.

This makes no sense.  Sector damage spills over to land units that way
since Chainsaw 3 added them, and to planes since 4.0.9.

Change sectdamage() not to damage land units and planes loaded on
ships or land units.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>