KMain()
    includes ::/StartOS.CC
        includes ~/MakeHome.CC
            includes ~/HomeSys.CC
                calls StartUpTasks()
                    includes ~/Once.CC
                        calls OnceExe().

ZOnce() appends src code to ~/Registry.CC Once/System tree, executed at next boot by System.

Once()  appends src code to ~/Registry.CC Once/User tree, executed at next boot by first User term.

At boot, OnceExe(), executes Once/System tree, ZOnceFlush()s it,
executes Once/User tree and OnceFlush()s.