Increase chance of gas stations spawning outside of towns

A couple things going on made gas stations very rare. First, gas stations and motels both share the "by_hiway" location, which is a hard location to nail a spot down in the first place.  Second, motels had minimum required per overmap and gas stations didn't, which meant that mapgen would favor motels over gas stations until at least 2 motels were placed.  This should put the selection of motels and gas stations on even footing and make it more likely for gas stations to spawn.