navigate: Require all ships to start in the same sector

navigate: Require all ships to start in the same sector

The capability to navigate ships spread over several sectors is
obscure and rarely useful.  Accidental use is probably more frequent
than intentional use.  Issues:

* Interactive prompts show only the flagship's position, and give no
  clue that some ships are actually elsewhere.

* Path finding is supported only when all navigating ships are in
  the same sector.

* Interdiction becomes rather complex.  For each movement, every
  sector entered is interdicted independently.  This means the same
  fort, ship, land unit or plane can interdict multiple times.
  Interdiction order depends on the order the code examines
  ships. which the player can control.  This is all pretty much
  undocumented.

* Complicates the code and its maintenance.  Multiplies the number of
  test cases needed to cover navigate.

I feel we're better off without this feature.

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