(ask_olist): Let land units attack as long as they have positive

(ask_olist): Let land units attack as long as they have positive
mobility, except for high-mobility terrain (mountains), where the
rules remain as they were: land units need to have all the mobility
charged for the attack, not counting combat and moving in to occupy.
Rationale: Making sure your land units reach attack positions with
enough mobility left is a pain in the neck.  Requiring only positive
mobility is friendlier, but allows rushing of mountains.