pgsql: Fix SPI cursor support to allow scanning the results of utility

Log Message:
-----------
Fix SPI cursor support to allow scanning the results of utility commands
that return tuples (such as EXPLAIN).  Per gripe from Michael Fuhr.
Side effect: fix an old bug that unintentionally disabled backward scans
for all SPI-created cursors.

Modified Files:
--------------
    pgsql/src/backend/executor:
        spi.c (r1.133 -> r1.134)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/spi.c.diff?r1=1.133&r2=1.134)
    pgsql/src/backend/tcop:
        pquery.c (r1.90 -> r1.91)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/pquery.c.diff?r1=1.90&r2=1.91)