Fix ship interdiction targeting ships not at sea

Fix ship interdiction targeting ships not at sea

perform_mission() needs to know whether it is targeting ships or
something else, because the rules differ: submarines interdict only
ships, land units get their damage reduced when interdicting ships,
and different news are generated.

The old code assumed it was targeting ships when the target sector was
sea.  Wrong when interdicting ships in harbors, bridges and such.
This has always been broken.  Except when checking a submarine's
target: there it tested argument s, which is gross, but at least it
works.  That code was added in v4.0.8.

Replace the broken test by the gross hack everywhere.  This fixes news
and damage from land units when ships get interdicted in non-sea
sectors.