Fix a sign error in conversion of HW time to local time, which caused
the jitter to be amplified instead of reduced. NTP with HW timestamping
should now be more stable and able to ignore occasionally delayed
readings of PHC.
It was never used for anything and messages in debug output already
include filenames, which can be easily grepped if there is a need
to see log messages only from a particular file.
For now, when converting a raw timestamp, return error of the last
sample as the maximum error of the timestamp. This is needed to include
the PHC reading delay in the NTP dispersion.
