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.

Tags:
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REL9_0_STABLE

Modified Files:
--------------
    pgsql/src/backend/parser:
        parse_expr.c (r1.256 -> r1.256.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_expr.c?r1=1.256&r2=1.256.2.1)
        parse_func.c (r1.224 -> r1.224.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_func.c?r1=1.224&r2=1.224.2.1)
        parse_oper.c (r1.113 -> r1.113.4.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_oper.c?r1=1.113&r2=1.113.4.1)
    pgsql/src/include/parser:
        parse_func.h (r1.68 -> r1.68.6.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/parser/parse_func.h?r1=1.68&r2=1.68.6.1)