Make the time stamp resolution per-packet.

Pcap-ng files don't have a per-file time stamp resolution, they have a
per-interface time stamp resolution.  Add new time stamp resolution
types of "unknown" and "per-packet", add the time stamp resolution to
struct wtap_pkthdr, have the libwiretap core initialize it to the
per-file time stamp resolution, and have pcap-ng do the same thing with
the resolution that it does with the packet encapsulation.

Get rid of the TS_PREC_AUTO_XXX values; just have TS_PREC_AUTO, which
means "use the packet's resolution to determine how many significant
digits to display".  Rename all the WTAP_FILE_TSPREC_XXX values to
WTAP_TSPREC_XXX, as they're also used for per-packet values.

Change-Id: If9fd8f799b19836a5104aaa0870a951498886c69
Reviewed-on: https://code.wireshark.org/review/4349
Reviewed-by: Guy Harris <guy@alum.mit.edu>