Implement TIMEZONE_HOUR, TIMEZONE_MINUTE per SQL92 specs.

Implement TIMEZONE_HOUR, TIMEZONE_MINUTE per SQL92 specs.
Check for and properly ignore FOREIGN KEY column constraints
 (already had fixed same for table constraints).
Define USER as synonym for CURRENT_USER per SQL92 specs
 (no longer allowed as bare column name, sorry).
Re-enable HAVING clause but no fixes elsewhere yet.
Other stuff from today's update of gram.y...