Fix march not to wipe out concurrent updates

Fix march not to wipe out concurrent updates

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

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

* Deity loads land unit onto a ship or land unit

* Land unit's crew killed just right, e.g. by collateral damage from
  interdiction, followed by additional updates, such as shell fire
  damage

* Sector no longer owned or allied, e.g. allied sector captured by an
  enemy (own sector would kill or retreat the land unit)

Writing a stale copy wipes out the updates that made the copy stale,
and triggers a seqno mismatch oops.  For instance, damage that follows
killing of all crew by collateral damage from interdiction is wiped
out.  If no damage follows, we still get a generation oops.