Add new DEBUG_LOG macro for debug messages. The messages are enabled
when compiled with --enable-debug and they are printed when the -d
option is used twice.
Implement a driver which allows using PTP hardware clock (PHC) as a
reference clock. It uses the PTP_SYS_OFFSET ioctl or clock_gettime()
to measure the offset between the PTP clock and the system clock. Ten
readings are made for every driver poll and the fastest one is returned.
As PHCs are typically kept in TAI instead of UTC, it's necessary to set
the TAI/UTC offset manually by the offset option. This could be improved
by obtaining the offset automatically from the right/UTC timezone.
Combine only sources whose distance is shorter than distance of the
selected source multiplied by the value of combinelimit and their
estimated frequencies are close to the frequency of the selected source.
Add outlyer status for sources which are selectable, but not included in
the combining. The status is displayed as '-' in the chronyc sources
output.
