pgsql: Tweak new error messages to match the actual syntax of DECLARE

Log Message:
-----------
Tweak new error messages to match the actual syntax of DECLARE CURSOR.
(Last night I copied-and-pasted from the WITH HOLD case, but that's
wrong because of the bizarrely irregular syntax specified by the standard.)

Modified Files:
--------------
pgsql/src/backend/executor:
spi.c (r1.182 -> r1.183)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/spi.c?r1=1.182&r2=1.183)
pgsql/src/backend/parser:
analyze.c (r1.368 -> r1.369)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/analyze.c?r1=1.368&r2=1.369)