retreat: Fix group retreat after failed board sinks ship

retreat: Fix group retreat after failed board sinks ship

Group retreat still doesn't work, because when boar() passes a sunk
ship to retreat_ship(), its owner has been reset to POGO already.
This makes it impossible to find the group to retreat.  Instead, it
attempts to retreat ships that sank in the same sector with group
retreat orders and with the same fleet letter assigned.  If any exist,
shp_may_nav() oopses, and prevents actual retreat of these ghosts.

The other retreat conditions don't have this problem, because they
call putship(), which resets the owner, only after retreat_ship().

Making boar() work the same is not practical.  Instead, add an owner
parameter to retreat_ship(), and for symmetry also to retreat_land().

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