SITTING in the White House one morning in late August 1967, President Johnson had the sudden idea that housing for the poor might be readily built on surplus federal land in the cities. He turned to assistant, Joseph A. Califano, Jr., and instructed him to begin, here and now, in Washington, D.C.  This was the start of the Fort Lincoln project, and of a national program of surplus land development for which Fort Lincoln was supposed to show the way.