tend ltend: Reject foreign tenders and land units more nicely

tend ltend: Reject foreign tenders and land units more nicely

We silently ignore foreign ships and land units.  This may leave the
player guessing why the command did nothing.  Report explicitly named
ones like this

    You don't own ship #160!

except for tend's target ships.  Nice reporting is a bit more involved
there, because you can tend to foreign target ships as long as they're
friendly.  Left for later.

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