Fix WHERE CURRENT OF to work as designed within plpgsql. The argument

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.