navigate march: Fix abort not to wipe out concurrent updates

navigate march: Fix abort not to wipe out concurrent updates

When the player aborts the command at the movement prompt, we write
back stale ships or land units, triggering a generation oops.  Any
updates made by other threads meanwhile are wiped out, triggering a
seqno mismatch oops.

Broken in commit 24000b4, v4.3.33.  Fix by restoring the lost
shp_nav_stay_behind() and lnd_mar_stay_behind() calls.

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