net/pcap: support Tx nanosecond timestamps

net/pcap: support Tx nanosecond timestamps

When capturing packets into a PCAP file, DPDK currently uses
microseconds for the timestamps. But libpcap supports interpreting
tv_usec as nanoseconds depending on the file timestamp precision,
as of commit ba89e4a18e8b ("Make timestamps precision configurable").

To support this, use PCAP_TSTAMP_PRECISION_NANO when creating the
empty PCAP file as specified by PCAP_OPEN_DEAD(3PCAP) and implement
nanosecond timeval addition. This also ensures that the precision
reported by capinfos is nanoseconds (9).

Note that NSEC_PER_SEC is defined as 1000000000L instead of 1e9 since
the latter might be interpreted as floating point.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>