From: Stephen Hemminger <shemminger@osdl.org>

The original problem all this is solving is that when NTP is slowing the clock
there existed real cases where time appeared to go backwards. Assuming NTP was
slowing the clock, then it would update the xtime by 999us at the next timer interrupt.
If a program read time three times:

A:	    xtime = t0
B: A+1000   xtime = t0 + 1000
C: B+1	    xtime = t0 + 999

To behave correctly C > B > A; but we were returning C < B

The code does have bug if we are losing clock interrupts.  The test for
lost interrupts needs to be after the interval clamp.