Elmer Kelton writes of his beloved home country of West Texas in these two novels of cowmen and cow country.   InPecos Crossing, two young cowboys, Johnny Fristo and Speck Quitman, have been cheated of six months' hard-earned salary by their rancher boss Larramore and intend getting what is due to them.   InShotgun, Texas rancher Blair Bishop has to contend with a rival cowman who is turning his herd loose on Bishop's land, and with a mean customer named Macy Modock, who Bishop sent to prison ten years past. Modock is out of the hoosegow and has returned determined to get even with the man who sent him up the river.