pgsql: Add support for input and output of interval values formatted per

Log Message:
-----------
Add support for input and output of interval values formatted per ISO 8601;
specifically, we can input either the "format with designators" or the
"alternative format", and we can output the former when IntervalStyle is set
to iso_8601.

Ron Mayer

Modified Files:
--------------
pgsql/doc/src/sgml:
config.sgml (r1.194 -> r1.195)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.194&r2=1.195)
datatype.sgml (r1.233 -> r1.234)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/datatype.sgml?r1=1.233&r2=1.234)
pgsql/src/backend/utils/adt:
datetime.c (r1.197 -> r1.198)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/datetime.c?r1=1.197&r2=1.198)
nabstime.c (r1.157 -> r1.158)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/nabstime.c?r1=1.157&r2=1.158)
timestamp.c (r1.194 -> r1.195)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/timestamp.c?r1=1.194&r2=1.195)
pgsql/src/backend/utils/misc:
guc.c (r1.476 -> r1.477)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.476&r2=1.477)
pgsql/src/bin/psql:
tab-complete.c (r1.175 -> r1.176)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/tab-complete.c?r1=1.175&r2=1.176)
pgsql/src/include:
miscadmin.h (r1.204 -> r1.205)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/miscadmin.h?r1=1.204&r2=1.205)
pgsql/src/include/utils:
datetime.h (r1.70 -> r1.71)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/datetime.h?r1=1.70&r2=1.71)
pgsql/src/test/regress/expected:
interval.out (r1.24 -> r1.25)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/interval.out?r1=1.24&r2=1.25)
pgsql/src/test/regress/sql:
interval.sql (r1.16 -> r1.17)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/interval.sql?r1=1.16&r2=1.17)