gpg: Show the default PIN hint also before "name" and "key-attr"

* g10/card-util.c (USER_PIN_DEFAULT): Move to the top.  Same for the
other constants.
(show_pin_hint): New.
(generate_card_keys): Use show_pin_hint.
(do_change_keyattr): Also show pin hint here.
(change_name): And here.

We used to show a hint for the default PINs only before generate.
However it is often useful to first change the attributes and thus the
hint should be show here as well.

The above is only done if no name has yet been set, thus before
setting the name we also show the hint.