pgsql: Add string_agg aggregate functions.

Log Message:
-----------
Add string_agg aggregate functions. The one argument version concatenates
the input values into a string. The two argument version also does the same
thing, but inserts delimiters between elements.

Original patch by Pavel Stehule, reviewed by David E. Wheeler and me.

Modified Files:
--------------
pgsql/doc/src/sgml:
func.sgml (r1.498 -> r1.499)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.498&r2=1.499)
pgsql/src/backend/utils/adt:
varlena.c (r1.174 -> r1.175)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/varlena.c?r1=1.174&r2=1.175)
pgsql/src/include/catalog:
catversion.h (r1.581 -> r1.582)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.581&r2=1.582)
pg_aggregate.h (r1.70 -> r1.71)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_aggregate.h?r1=1.70&r2=1.71)
pg_proc.h (r1.566 -> r1.567)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h?r1=1.566&r2=1.567)
pgsql/src/include/utils:
builtins.h (r1.345 -> r1.346)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/builtins.h?r1=1.345&r2=1.346)
pgsql/src/test/regress/expected:
aggregates.out (r1.20 -> r1.21)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/aggregates.out?r1=1.20&r2=1.21)
pgsql/src/test/regress/sql:
aggregates.sql (r1.16 -> r1.17)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/aggregates.sql?r1=1.16&r2=1.17)