pcap-file: Add nanosecond resolution pcap support.

pcap-file: Add nanosecond resolution pcap support.

PCAP header magic numbers are different for microsecond and nanosecond
resolution timestamps. This patch adds support for understanding the
difference and reporting the time correctly with ovs_pcap_read().

When writing pcap files, OVS will always use microsecond resolution, so
no new calculations were added to those functions.

Signed-off-by: Mark Michelson <mmichels@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>