pgsql: Fix SPI_cursor_open() and SPI_is_cursor_plan() to push the SPI

Log Message:
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Fix SPI_cursor_open() and SPI_is_cursor_plan() to push the SPI stack before
doing anything interesting, such as calling RevalidateCachedPlan(). The
necessity of this is demonstrated by an example from Willem Buitendyk:
during a replan, the planner might try to evaluate SPI-using functions,
and so we'd better be in a clean SPI context.

A small downside of this fix is that these two functions will now fail
outright if called when not inside a SPI-using procedure (ie, a
SPI_connect/SPI_finish pair). The documentation never promised or suggested
that that would work, though; and they are normally used in concert with
other functions, mainly SPI_prepare, that always have failed in such a case.
So the odds of breaking something seem pretty low.

In passing, make SPI_is_cursor_plan's error handling convention clearer,
and fix documentation's erroneous claim that SPI_cursor_open would
return NULL on error.

Before 8.3 these functions could not invoke replanning, so there is probably
no need for back-patching.

Modified Files:
--------------
pgsql/doc/src/sgml:
spi.sgml (r1.59 -> r1.60)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/spi.sgml?r1=1.59&r2=1.60)
pgsql/src/backend/executor:
spi.c (r1.187 -> r1.188)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/spi.c?r1=1.187&r2=1.188)