pgsql: Correct/improve the datetime_precision field in the information

Log Message:
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Correct/improve the datetime_precision field in the information schema.
In particular, always show 0 for the date type instead of null, and show
6 (the default) for time, timestamp, and interval without a declared
precision. This is now in fuller conformance with the SQL standard.

Also clarify the documentation about this.

discovered and analyzed by Konstantin Izmailov and Tom Lane

Modified Files:
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pgsql/doc/src/sgml:
information_schema.sgml (r1.38 -> r1.39)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/information_schema.sgml?r1=1.38&r2=1.39)
pgsql/src/backend/catalog:
information_schema.sql (r1.53 -> r1.54)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/information_schema.sql?r1=1.53&r2=1.54)