march: Fix check for sector abandonment

march: Fix check for sector abandonment

sct_prewrite() makes an owned sector revert to the deity when there
are no civilians, military or own land units.

would_abandon() tries to predict that, but gets it wrong: it ignores
land units that evade spy detection or are loaded on ships, and it
fails to ignore land units loaded on land units marching out.

Broken in commit 7c1b166, v4.3.33.  Fix by counting manually rather
than with unitsatxy().

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