pgsql: Remove the single-argument form of string_agg().

Log Message:
-----------
Remove the single-argument form of string_agg(). It added nothing much in
functionality, while creating an ambiguity in usage with ORDER BY that at
least two people have already gotten seriously confused by. Also, add an
opr_sanity test to check that we don't in future violate the newly minted
policy of not having built-in aggregates with the same name and different
numbers of parameters. Per discussion of a complaint from Thom Brown.

Modified Files:
--------------
pgsql/doc/src/sgml:
func.sgml (r1.523 -> r1.524)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.523&r2=1.524)
syntax.sgml (r1.149 -> r1.150)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/syntax.sgml?r1=1.149&r2=1.150)
pgsql/src/backend/utils/adt:
varlena.c (r1.177 -> r1.178)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/varlena.c?r1=1.177&r2=1.178)
pgsql/src/include/catalog:
catversion.h (r1.589 -> r1.590)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.589&r2=1.590)
pg_aggregate.h (r1.71 -> r1.72)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_aggregate.h?r1=1.71&r2=1.72)
pg_proc.h (r1.574 -> r1.575)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h?r1=1.574&r2=1.575)
pgsql/src/include/utils:
builtins.h (r1.352 -> r1.353)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/builtins.h?r1=1.352&r2=1.353)
pgsql/src/test/regress/expected:
aggregates.out (r1.22 -> r1.23)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/aggregates.out?r1=1.22&r2=1.23)
opr_sanity.out (r1.90 -> r1.91)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/opr_sanity.out?r1=1.90&r2=1.91)
pgsql/src/test/regress/sql:
aggregates.sql (r1.18 -> r1.19)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/aggregates.sql?r1=1.18&r2=1.19)
opr_sanity.sql (r1.73 -> r1.74)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/opr_sanity.sql?r1=1.73&r2=1.74)