subs: Split lnd_mar_put() off lnd_put() and specialize

subs: Split lnd_mar_put() off lnd_put() and specialize

lnd_put() serves two masters: march, which wants it to report
"stopped" and write back struct ulist member mobility to
unit.land.lnd_mobil, and ground combat, which doesn't.

lnd_put() assumes march when actor is non-zero.  Correct (but see
commit 8c502d4).  Dates back to Empire 2.

Too ugly for my taste.  Specialize for each master instead:
lnd_mar_put() for march, and lnd_put() for ground combat.

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