Fix navigate and march not to lay mines free of charge
do_unit_move() reads the ships into a list. It re-reads them when it
prompts for sub-commands. shp_nav_one_sector() writes them back when
it moves ships.
Mine-laying (sub-command 'd') updates the minelayer, invalidating the
copy in the list. Any movement sub-command before the next prompt for
sub-commands wiped out this update, triggering a seno mismatch oops.
Happens only if 'd' is used without arguments, because remaining
sub-commands are discarded when there are arguments.
Broken when mine-laying was added in commits
2438fe7c, v4.3.7.
Same for march, commit
274c8e42, v4.3.7.
Fix by stopping after 'd' regardless of arguments.
(cherry picked from commit
28cc236e1264d57a06dfb89cf43a600ba88d271c)