Support nanosecond-resolution time stamps when writing pcap-ng files.

Pcap-ng files have supported variable time stamp resolutions since Day
One, and any code that reads those files but can't handle
nanosecond-resolution time stamps is broken.

Pcap files got a separate magic number for nanosecond-resolution files
relatively recently, and not all code that reads pcap files handles that
magic number, so we don't support it for pcap files.

Based on https://code.wireshark.org/review/#/c/4304/ from Peter Hatina.

Change-Id: I4f4e3b5ca0ba4fe9394765edb54bb77a4a776237
Reviewed-on: https://code.wireshark.org/review/4358
Reviewed-by: Guy Harris <guy@alum.mit.edu>