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_4_STABLE

Modified Files:
--------------
    pgsql/src/backend/parser:
        parse_expr.c (r1.241.2.5 -> r1.241.2.6)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_expr.c?r1=1.241.2.5&r2=1.241.2.6)
        parse_func.c (r1.216 -> r1.216.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_func.c?r1=1.216&r2=1.216.2.1)
        parse_oper.c (r1.109 -> r1.109.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_oper.c?r1=1.109&r2=1.109.2.1)
    pgsql/src/include/parser:
        parse_func.h (r1.65 -> r1.65.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/parser/parse_func.h?r1=1.65&r2=1.65.2.1)