Make land unit attack mobility cost consistent with march cost:

Make land unit attack mobility cost consistent with march cost:
(att_mobcost): New.
(ask_olist, take_move_in_mob): Use it.  Attacking land units can now
use roads and suffer the newly taken penalty.  No difference in most
cases, because the penalty commonly cancels the road bonus.
(get_mob_support, calc_mobcost, ask_move_in_off): Use it.  No
functional change now; ensures that military's attack mobility cost
will stay consistent with move cost.
(MOB_NONE): Unused, remove.
(sector_mcost): Simplify.