pgsql: Allow SQL-language functions to return the output of an

Log Message:
-----------
Allow SQL-language functions to return the output of an INSERT/UPDATE/DELETE
RETURNING clause, not just a SELECT as formerly.

A side effect of this patch is that when a set-returning SQL function is used
in a FROM clause, performance is improved because the output is collected into
a tuplestore within the function, rather than using the less efficient
value-per-call mechanism.

Modified Files:
--------------
pgsql/doc/src/sgml:
xfunc.sgml (r1.132 -> r1.133)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xfunc.sgml?r1=1.132&r2=1.133)
pgsql/src/backend/executor:
execQual.c (r1.235 -> r1.236)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execQual.c?r1=1.235&r2=1.236)
functions.c (r1.126 -> r1.127)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/functions.c?r1=1.126&r2=1.127)
pgsql/src/backend/tcop:
dest.c (r1.72 -> r1.73)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/dest.c?r1=1.72&r2=1.73)
pgsql/src/backend/utils/fmgr:
README (r1.15 -> r1.16)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/fmgr/README?r1=1.15&r2=1.16)
pgsql/src/include/executor:
functions.h (r1.31 -> r1.32)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/functions.h?r1=1.31&r2=1.32)
pgsql/src/include/nodes:
execnodes.h (r1.193 -> r1.194)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/execnodes.h?r1=1.193&r2=1.194)
pgsql/src/include/tcop:
dest.h (r1.54 -> r1.55)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/tcop/dest.h?r1=1.54&r2=1.55)
pgsql/src/test/regress/expected:
rangefuncs.out (r1.19 -> r1.20)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/rangefuncs.out?r1=1.19&r2=1.20)
pgsql/src/test/regress/output:
create_function_1.source (r1.33 -> r1.34)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/output/create_function_1.source?r1=1.33&r2=1.34)
pgsql/src/test/regress/sql:
rangefuncs.sql (r1.8 -> r1.9)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/rangefuncs.sql?r1=1.8&r2=1.9)