pgsql: Fix WHERE CURRENT OF to work as designed within plpgsql.

Log Message:
-----------
Fix WHERE CURRENT OF to work as designed within plpgsql. The argument
can be the name of a plpgsql cursor variable, which formerly was converted
to $N before the core parser saw it, but that's no longer the case.
Deal with plain name references to plpgsql variables, and add a regression
test case that exposes the failure.

Modified Files:
--------------
pgsql/src/backend/executor:
execCurrent.c (r1.13 -> r1.14)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execCurrent.c?r1=1.13&r2=1.14)
pgsql/src/backend/parser:
gram.y (r2.688 -> r2.689)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y?r1=2.688&r2=2.689)
parse_expr.c (r1.247 -> r1.248)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_expr.c?r1=1.247&r2=1.248)
pgsql/src/test/regress/expected:
plpgsql.out (r1.79 -> r1.80)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/plpgsql.out?r1=1.79&r2=1.80)
pgsql/src/test/regress/sql:
plpgsql.sql (r1.66 -> r1.67)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/plpgsql.sql?r1=1.66&r2=1.67)