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:
----
REL7_4_STABLE

Modified Files:
--------------
    pgsql/src/backend/parser:
        parse_expr.c (r1.163.2.5 -> r1.163.2.6)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_expr.c?r1=1.163.2.5&r2=1.163.2.6)
        parse_func.c (r1.161 -> r1.161.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_func.c?r1=1.161&r2=1.161.2.1)
        parse_oper.c (r1.76 -> r1.76.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_oper.c?r1=1.76&r2=1.76.2.1)
    pgsql/src/include/parser:
        parse_func.h (r1.50 -> r1.50.4.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/parser/parse_func.h?r1=1.50&r2=1.50.4.1)