convert shoot: Fix to ignore foreign and embarked land units

convert shoot: Fix to ignore foreign and embarked land units

Convert and shoot put land units to work regardless of ownership.
Clearly wrong for foreign, non-allied land units trapped in the
sector.

They also happily use land units loaded on ships or land units, unlike
most other commands.  Questionable, because unload need not be
possible.

Checking for ownership and load status was neglected when land units
were added in Chainsaw 3.  Non-allied land units became possible in
4.0.0.

Checking relations and whether unload would be possible is not worth
the trouble here, simply ignore all foreign and embarked land units.

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