Make the nativity limits on the source tile for regular attacks consistent

Make everything test if the attacking unit can exist on its tile like
unit_move_to_tile_test() already did. This also makes the limits on the source
tile of regular attacks consistent with those on spy actions.

See bug #22381

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25730 a0f10bec-cc02-0410-94fc-a9cfff90b4cd