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:
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REL8_3_STABLE

Modified Files:
--------------
    pgsql/src/backend/parser:
        parse_expr.c (r1.226.2.4 -> r1.226.2.5)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_expr.c?r1=1.226.2.4&r2=1.226.2.5)
        parse_func.c (r1.201 -> r1.201.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_func.c?r1=1.201&r2=1.201.2.1)
        parse_oper.c (r1.101 -> r1.101.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_oper.c?r1=1.101&r2=1.101.2.1)
    pgsql/src/include/parser:
        parse_func.h (r1.59 -> r1.59.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/parser/parse_func.h?r1=1.59&r2=1.59.2.1)