pgsql: Change SPI functions to use a `long' when specifying the number

Log Message:
-----------
Change SPI functions to use a `long' when specifying the number of tuples
to produce when running the executor. This is consistent with the internal
executor APIs (such as ExecutorRun), which also use a long for this purpose.
It also allows FETCH_ALL to be passed -- since FETCH_ALL is defined as
LONG_MAX, this wouldn't have worked on platforms where int and long are of
different sizes. Per report from Tzahi Fadida.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        spi.sgml (r1.40 -> r1.41)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/spi.sgml.diff?r1=1.40&r2=1.41)
    pgsql/src/backend/executor:
        spi.c (r1.138 -> r1.139)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/spi.c.diff?r1=1.138&r2=1.139)
    pgsql/src/include/executor:
        spi.h (r1.51 -> r1.52)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/spi.h.diff?r1=1.51&r2=1.52)
    pgsql/src/pl/plpgsql/src:
        pl_exec.c (r1.136 -> r1.137)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.136&r2=1.137)
    pgsql/src/pl/plpython:
        plpython.c (r1.60 -> r1.61)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/plpython.c.diff?r1=1.60&r2=1.61)