navigate march: Stop on non-fatal mine hits, too

navigate march: Stop on non-fatal mine hits, too

We stop on mine hits only when they're fatal.  Has always been that
way.  When interdiction was added in Chainsaw, it worked the same.
Empire 2 changed the commands to stop on any interdiction damage.  Now
stop on any mine damage, too.

Interdiction can fail to do damage (all bombs miss), and mines can be
detected without damage (by sweeping).  Perhaps we should stop then as
well.  Left for another day.

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