Add support for getting nanosecond time stamp resolution when capturing.

If we have pcap_set_tstamp_precision(), use it to request nanosecond
time stamp resolution *if* we're writing a pcap-ng file; any code that
reads those files and can't handle nanosecond time stamp resolution is
broken and needs to be fixed.

If we're writing a pcap file, don't ask for nanosecond resolution time
stamps, as that requires a different magic number for pcap files, and
not all code that reads pcap files can handle that.  (Unlike pcap-ng,
where the ability to have non-microsecond time stamp resolution was
present from Day One, it's a relatively recent addition to pcap.)  We
could add a command-line option/GUI option for that, like the option
recent versions of tcpdump have, if it matters.

Change-Id: I8fa464eb929feecb9a70be70712502c9f0cc5270
Reviewed-on: https://code.wireshark.org/review/4355
Reviewed-by: Guy Harris <guy@alum.mit.edu>