epicsThreadClockGetResolution

Consider adding a routine 
    double epicsThreadClockGetResolution();
It would return the clock tick rate where by clock we mean the thing that
is like the vxWorks sysClockRateGet().


The implementation should be:
    Provide a default that determines the rate emperically at int time by
    calling epicsThreadSleep for amaller and smaller delays until the actual
    delay no longer decreases.
    Each os/xxx can override the default if desired.