[Sccs-devel] Bugs handling UTC in 5.08

[Bruce L. <bru...@gm...>]

This started as a bug-fix for prs.c, which wouldn't compile with
-DGMT_TIME.  During testing, I found several related bugs, and added some
related features which I've been using locally for some time.  These may
help with v6.

Related bugs fixed:

 * Incorrect time (not corrected for zone offset) resulting in warnings
   about retrieval time being older than delta time (noticeable West of the
   Prime Meridian).

Related features added:

 * %N% and %O% keywords added and documented.

   These are for current date-time and last applied delta date-time,
   respectively.  Format is standard date-time format as documented in:
   1. CCSDS 301.0-B-4 section 3.5 type A and B formats
       * http://public.ccsds.org/publications/archive/301x0b4e1.pdf
   2. EDSC EX000013.1
       * Http://www.exchangenetwork.net/standards/
         Rep_Date_Time_01_06_2006_Final.pdf
   3. RFC 3339
       * https://www.ietf.org/rfc/rfc3339.txt
   Note: all of the above are freely available at the URIs provided.  They
   are all based on ISO 8601 (which is not freely available and is
   expensive).

   Examples:
       %N% 2015-10-11T17:55:49.104271752Z
       %O% 2015-10-11T11:45:16Z

 * Improved parsing of date and time.

   In case XPG decides to allow or require standard format date-time in the
   s-file.

Incidentally, "GMT" is ambiguous and is considered archaic; better to use
"UTC".  Refer to:

 * http://www.ucolick.org/~sla/leapsecs/timescales.html
   and the references therein, especially:
 * http://articles.adsabs.harvard.edu/full/1978QJRAS..19..290S

Patch attached.