Factor out common land unit fire code into lnd_fire()
This takes care of a number of bugs / inconsistencies:
* Resupply before fire: fire command did not require unit to be in
supply, and resupplied shells. Everywhere else (return fire,
support and interdiction) the land unit had to be in supply after
resupply of everything. Unify not to resupply anything and not to
require being in supply. This is consistent with ships and sectors.
* Resupply after fire: fire command resupplied shells after active
fire. Unify not to do that. This is consistent with ships and
sectors.
* When a land unit returned fire to multiple attackers, quiet_bigdef()
charged it ammo for each one. Finally, it was charged one shell
more by use_ammo(). Except only the first land unit got charged
there in fact, because buggy add_to_fired_queue() entered only the
first land unit into the defender list. Fix add_to_fired_queue()
and change quiet_bigdef() not to charge ammo, just like for ships
and sectors. This charges only one shell instead of the true ammo
use, which is wrong, but consistent with ships.
* lnd_support() tallied support damage unrounded. Unify to round
before tally.