Getting useful Rx timestamps for packet capture

[Stephen Hemminger]

Current DPDK support for timestamp is device dependent.
This is bad/broken for things like packet capture which
could be using the timestamp to get more accurate data into
the capture file.

Linux network timestamps are in timespec which is in nano
seconds since 1/1/1970. Pcapng typically uses 64 bit nanoseconds
since 1/1/1970.

It would be best if the drivers all did the conversion to
a standard format (ns since 1/1/1970). If the value read from
the hardware is in other units, then it should be scaled
in the PMD