Fix navigate and march not to prompt for number of mines to lay

Fix navigate and march not to prompt for number of mines to lay

When sub-command 'd' was used without arguments, do_unit_move() failed
to supply the second argument to mine(), which duly prompted for it.
This contracticted info, and could trigger a generation oops.

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.

The mine prompt made the list stale.  Movement sub-commands before the
next prompt for sub-commands wrote back stale ships, triggering a
generation oops.  Example: "nav 15 dg".

Broken when mine-laying was added in commits 2438fe7c, v4.3.7.

Same for march, commit 274c8e42, v4.3.7.