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)