update: Rearrange code to pay non-sector military

update: Rearrange code to pay non-sector military

Split upd_slmilcosts() into prep_ships() and prep_lands().  Move the
sanity check for dead ships and land units from prod_ships() and
prod_lands() there.  Move their call from prepare_sects() to its
caller, along with pay_reserve().

Create prep_planes() for symmetry.  Pilots are now paid at the same
time as other military.  Can matter only when the country goes broke
during the update.

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