retreat: Rewrite automatic retreat code to fix its many bugs

retreat: Rewrite automatic retreat code to fix its many bugs

Much of the retreat code duplicates navigate and march code.  Worse,
retreat's version is full of bugs:

* Land units can sometimes retreat when they couldn't march: while on
  the trading block (forbidden with march since 4.0.9), crewless
  (likewise since 4.0.0), kidnapped in a foreign sector (inconsistent
  since land units were added in Chainsaw 3), loaded on a ship
  (likewise) or a land unit (inconsistent since trains were added in
  4.0.0).

* Ships can retreat while on the trading block (forbidden with
  navigate since 4.0.9)

* Land units can't retreat into foreign sectors even though they could
  march there, namely when sector is allied or the land unit is a spy.
  They can march there since 4.0.0.

* Land units keep their fortification on retreat.  Has been that way
  since retreat was added in Chainsaw.

Then there's group retreat.  It's basically crazy:

* It triggers retreat for everyone in the same fleet or army, one
  after the other, regardless of retreat path, conditions (including
  group retreat), or even location.  The latter is quite abusable
  since retreats aren't interdicted.  Has been that way since retreat
  was added in Chainsaw.

* Group retreat fails to trigger when the originally retreating ship
  or land unit has no retreat path left when it's done.  Broken in
  commit b860123.

Finally, the reporting to the owner is sub-par:

* When a retreat is cut short by insufficient mobility or
  obstructions, its end sector isn't reported, leaving the player
  guessing.

* Non-retreats can be confusingly reported as retreat to the same
  sector.  Can happen when the retreat path starts with 'h' (obscure
  feature to suppress a single retreat), or when a group retreat
  includes a ship or land unit without retreat orders.

* Interaction with mines during retreat is reported before the retreat
  itself, which can be quite confusing.

* Sweeping landmines isn't reported at all.

* Much code and much bulletin text is dedicated to reporting what
  caused the retreat, even though it should be perfectly obvious.

Rewrite this on top of common navigate and march code.  Reuse of
common code fixes the "can retreat when it couldn't navigate/march"
and the "can't retreat into sectors it could navigate or march into"
bugs, and improves the reporting.

One special case isn't a bug fix but a rule change: mountains.  The
old code forbids that explicitly, and it's clearly intentional, if
undocumented.  The new code allows it by not doing anything special.

Turn group retreat into an actual group retreat: everyone in the same
fleet and sector with the the same retreat path and group retreat
condition joins the group.  The group retreats together, just like in
navigate and march.

Take care to always report the end sector.  When retreat is
impossible, report "can't retreat".  When retreat is partial, report
"and stays in X,Y".  When it's complete, report "stopped at X,Y".

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