date/time improvements for 7.2

Accept an INTERVAL argument for SET TIME ZONE per SQL99.Modified the parser and the SET handlers to use full Node
structuresratherthan simply a character string argument.
 
I've implemented and committed changes to improve the feature set for
INTERVAL, as well as making other bug fixes and improvements to other
date/time types. There is one more issue to address as a bug fix
regarding precision and rounding for INTERVAL values for which precision
has been specified (it is possible to pick up some extraneous cruft in
the lsb of the floating point number when rounding).

The CVS log entry is below. initdb required (sorry!) and non-reference
date/time regression tests probably will need to be updated.

All regression tests pass.
                          - Thomas

Implement INTERVAL() YEAR TO MONTH (etc) syntax per SQL99.Does not yet accept the goofy string format that goes along
with,butthat should be easy to add as a bug fix now or feature improvementlater.
 
Accept an INTERVAL argument for SET TIME ZONE per SQL99.Modified the parser and the SET handlers to use full Node
structuresratherthan simply a character string argument.
 
Implement INTERVAL() YEAR TO MONTH (etc) syntax per SQL99.Does not yet accept the goofy string format that goes along
with,butthis should be fairly straight forward to fix now as a bug or lateras a feature.
 
Implement precision for the INTERVAL() type.Use the typmod mechanism for both of INTERVAL features.
Fix the INTERVAL syntax in the parser:opt_interval was in the wrong place.
INTERVAL is now a reserved word, otherwise we get reduce/reduce errors.
Implement an explicit date_part() function for TIMETZ.Should fix coersion problem with INTERVAL reported by Peter E.
Fix up some error messages for date/time types.Use all caps for type names within message.
Fix recently introduced side-effect bug disabling 'epoch' as a
recognizedfield for date_part() etc. Reported by Peter E. (??)
Bump catalog version number.
Rename "microseconds" current transaction time fieldfrom ...Msec to ...Usec. Duh!
date/time regression tests updated for reference platform, but a fewchanges will be necessary for others.