(lnd_mobtype): New.

(lnd_mobtype): New.
(lnd_path): Use it.
(lnd_mobcost): Use it, remove last parameter.  Callers changed.  This
fixes mobility use of trains when retreating, both for retreat orders
and for failed morale checks.
(retreat_land1): Fix test for impassable terrain.  Before, trains
could retreat off rail.
(lnd_take_casualty): Test for impassable terrain.  Before, trains
could retreat off rail.