pgsql: Improved version of patch to protect pg_get_expr() against

Log Message:
-----------
Improved version of patch to protect pg_get_expr() against misuse:
look through join alias Vars to avoid breaking join queries, and
move the test to someplace where it will catch more possible ways
of calling a function. We still ought to throw away the whole thing
in favor of a data-type-based solution, but that's not feasible in
the back branches.

This needs to be back-patched further than 9.0, but I don't have time
to do so today. Committing now so that the fix gets into 9.0beta4.

Modified Files:
--------------
pgsql/src/backend/parser:
parse_expr.c (r1.256 -> r1.257)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_expr.c?r1=1.256&r2=1.257)
parse_func.c (r1.224 -> r1.225)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_func.c?r1=1.224&r2=1.225)
parse_oper.c (r1.113 -> r1.114)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_oper.c?r1=1.113&r2=1.114)
pgsql/src/include/parser:
parse_func.h (r1.68 -> r1.69)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/parser/parse_func.h?r1=1.68&r2=1.69)