pgsql: Add new return codes SPI_OK_INSERT_RETURNING etc to the SPI API.

Log Message:
-----------
Add new return codes SPI_OK_INSERT_RETURNING etc to the SPI API.
Fix all the standard PLs to be able to return tuples from FOO_RETURNING
statements as well as utility statements that return tuples.  Also,
fix oversight that SPI_processed wasn't set for a utility statement
returning tuples.  Per recent discussion.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        spi.sgml (r1.46 -> r1.47)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/spi.sgml.diff?r1=1.46&r2=1.47)
    pgsql/src/backend/executor:
        spi.c (r1.157 -> r1.158)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/spi.c.diff?r1=1.157&r2=1.158)
    pgsql/src/include/executor:
        spi.h (r1.54 -> r1.55)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/spi.h.diff?r1=1.54&r2=1.55)
    pgsql/src/pl/plperl:
        plperl.c (r1.117 -> r1.118)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/plperl.c.diff?r1=1.117&r2=1.118)
    pgsql/src/pl/plpgsql/src:
        pl_exec.c (r1.176 -> r1.177)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.176&r2=1.177)
    pgsql/src/pl/plpython:
        plpython.c (r1.85 -> r1.86)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/plpython.c.diff?r1=1.85&r2=1.86)
    pgsql/src/pl/tcl:
        pltcl.c (r1.106 -> r1.107)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/tcl/pltcl.c.diff?r1=1.106&r2=1.107)