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.

Completion of back-port of my patch of yesterday.

Tags:
----
REL8_0_STABLE

Modified Files:
--------------
pgsql/src/backend/parser:
parse_expr.c (r1.179.4.4 -> r1.179.4.5)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_expr.c?r1=1.179.4.4&r2=1.179.4.5)
parse_func.c (r1.175 -> r1.175.4.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_func.c?r1=1.175&r2=1.175.4.1)
parse_oper.c (r1.81 -> r1.81.4.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_oper.c?r1=1.81&r2=1.81.4.1)
pgsql/src/include/parser:
parse_func.h (r1.53 -> r1.53.4.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/parser/parse_func.h?r1=1.53&r2=1.53.4.1)