op.c: Factor out common entersub-building code

op.c: Factor out common entersub-building code

This same incantation occurs four times, though the readpipe variant
was slightly different.  The only difference was the lack of a scalar
flag on a null op, which makes no difference:

-           <1> ex-rv2cv sK ->-
5              <$> gv(*require) s ->6

-           <1> ex-rv2cv K ->-
5              <$> gv(*readpipe) s ->6

So I did not include the scalar() call, as it was unnecessary.  Also,
I changed op_append_elem to newLISTOP, since the former calls the lat-
ter anyway in these cases.