Introduce a helper function to identify valid (non-zero) time stamps.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Invoke the clock check even if the time stamp nanoseconds field is zero.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
config: Introduce options for correcting transmit and receive delays.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
port: correct transmit and receive time stamps for their calibrated delays.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>