pgsql: Remove the prohibition on executing cursor commands through

Log Message:
-----------
Remove the prohibition on executing cursor commands through SPI_execute.
Vadim had included this restriction in the original design of the SPI code,
but I'm darned if I can see a reason for it.

I left the macro definition of SPI_ERROR_CURSOR in place, so as not to
needlessly break any SPI callers that are checking for it, but that code
will never actually be returned anymore.

Modified Files:
--------------
pgsql/doc/src/sgml:
spi.sgml (r1.54 -> r1.55)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/spi.sgml.diff?r1=1.54&r2=1.55)
pgsql/src/backend/executor:
spi.c (r1.173 -> r1.174)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/spi.c.diff?r1=1.173&r2=1.174)
pgsql/src/include/executor:
spi.h (r1.59 -> r1.60)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/spi.h.diff?r1=1.59&r2=1.60)
pgsql/src/pl/plpgsql/src:
pl_exec.c (r1.190 -> r1.191)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.190&r2=1.191)