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 structures
rather than 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, but
that should be easy to add as a bug fix now or feature improvement
later.
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.