Stop readline($foo) from autovivifying

Stop readline($foo) from autovivifying

Currently <$foo> does not autovivify, but readline($foo) does, due to
the way pp_readline calls pp_rv2gv directly, with PL_op still holding
a pp_readline op, whose flags may have completely different meanings.

readline uses the OPf_SPECIAL flag to distinguish <$foo> from readline
($foo).  rv2gv uses it to determine whether to autovivify; hence the
discrepancy.