Getting useful Rx timestamps for packet capture

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