edit: Fix fortification limit of embarked planes and land units
We generally preserve the invariant "land units and missiles loaded on
a land unit or ship are never fortified / hardened": fortify and
harden refuse to touch embarked land units and missiles, load and
lload zap land unit fortification on load, and refuse to load hardened
missiles.
The exception is edit, which happily fortifies embarked land units
(edit u key 'F'), hardens embarked missiles (edit p key 'F'), loads
fortified land units (edit u keys 'S' and 'Y') and hardened planes
(edit p keys 's' and 'y').
Fix the first two by correcting the new value's upper limit to zero
for embarked land units and planes. The next commit will take care of
the rest.
The fix is visible in tests where test setup fortifies land units with
"edit u * F ?..." without excepting embarked ones.
Signed-off-by: Markus Armbruster <armbru@pond.sub.org>