Fix navigate not to wipe out concurrent updates

Fix navigate not to wipe out concurrent updates

Navigation code reads the ships into a ship list, and writes them back
when it changes them, e.g. when a ship stops.  If a ship changes in
the ship file while it is in such a ship list, the copy in the ship
list becomes stale, and must not be used.

To that end, do_unit_move() calls shp_nav() after prompting for path
or destination.  shp_nav() re-reads all the ships.  Unfortunately, it
still writes back stale copies in certain circumstances.  Known ways
to trigger such writes:

* Deity sets a sail path

* Ship's crew gone, e.g. killed by shell fire

* Sector no longer navigable, e.g. harbor shelled down, or bridge
  built

Writing a stale copy wipes out the updates that made the copy stale,
and triggers a seqno mismatch oops.  For instance, ship damage that
kills all crew while the ship is being navigated gets wiped out.