(nxtitem): Skipped unowned sectors (could not occur), ships, planes,

(nxtitem): Skipped unowned sectors (could not occur), ships, planes,
land units and nukes for selection type NS_LIST.  Thus, unowned items
explicitly selected by item number are skipped, while the same items
selected implicitely by group, area or whatever are not.  This makes
no sense.  Some callers break the nstr_item abstraction to prevent the
skipping, so they get all items regardless of how they were selected.
All other callers filter out unowned items.  This skipping misfeature
dates back to the very oldest known versions of Empire.  Remove it.
Also remove skipping prevention hacks from callers.

(tend_nxtitem): Now identical to nxtitem(), except it lacks some extra
normalization of item coordinates, which should have no effect.
Replace by nxtitem().