gpg: Print designated revokers also in a standard listing.

* g10/keylist.c (print_revokers): Add arg with_colon, adjust callers,
add human printable format.
(list_keyblock_print): Call print_revokers.

Designated revokers were only printed in --with-colons mode. For
quick inspection of a key it is useful to see them right away.

(cherry picked from commit 9d618d1273120ca2cca97028730352768b0c1897)