Fix INTERVAL output when year/month has different sign as day/hour etc.

 Previously, all fields were unsigned, with only a trailing "ago" to
 indicate negative intervals. Now, ISO format does not use "ago", and
 and the traditional PostgreSQL format has the first numeric field unsigned
 with "ago" supporting that field. So "1 month - 2 days ago" is two days
 less than a month in the past.
Fix interval arithmetic across daylight savings time boundaries.
 Previously, most math across boundaries introduced a one hour offset.
Allow some date/time functions to return NULL if called with NULL args.
Implement functions for AT TIME ZONE support.
Support "SAT" as an Australian time zone if USE_AUSTRALIAN_RULES
 is defined.