fire: Check land unit guns earlier, drop useless checks

fire: Check land unit guns earlier, drop useless checks

We check all necessary conditions for being able to fire before
prompting for a target.  Except for land unit guns.  Clean that up.

fort_fire(), shp_fire() or lnd_fire() fail only when the fort, ship or
land unit can't fire.  If that happens, our checking is incomplete.
Oops then.

We recheck some of the necessary conditions after getting the target.
However, because the command fails when the firing sector, ship or
land unit has changed since the first check, these rechecks can't
fail.  Drop them.

Note that the rechecks were just as useless before commit 66165f3
fixed fire to fail on change, because they rechecked the unchanged
cached copy instead of the possibly changed original.

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