Accept an INTERVAL argument for SET TIME ZONE per SQL99.

Accept an INTERVAL argument for SET TIME ZONE per SQL99.
 Modified the parser and the SET handlers to use full Node structures
 rather than 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, but
 this should be fairly straight forward to fix now as a bug or later
 as 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 recognized
 field for date_part() etc. Reported by Peter E. (??)
Bump catalog version number.
Rename "microseconds" current transaction time field
 from ...Msec to ...Usec. Duh!
date/time regression tests updated for reference platform, but a few
 changes will be necessary for others.