navigate march: Wipe mission and retreat orders less eagerly

navigate march: Wipe mission and retreat orders less eagerly

navi() uses shp_sel() to collect ships, then shp_nav() to drop
ineligible ships.  shp_sel() wipes mission and retreat orders.  Stupid
when shp_nav() will drop them right away.

Avoid that by having shp_sel() check shp_nav()'s conditions, too.

navi()'s shp_nav() call won't find anything to drop now.  The call
will be removed shortly.

This drops "& stays in" from some failure reports, since shp_nav()'s
reject messages end with "& stays in X,Y", and shp_sel()'s don't.

Likewise for marc(), lnd_sel(), lnd_mar().

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