gpg: Allow printing of MPI values in --list-mode.

* g10/parse-packet.c (set_packet_list_mode): Set mpi_print_mode.
* g10/misc.c (mpi_print): Do not print an extra leading zero.

This was in older versions possible using "--debug 4" but that was
disabled in 2.1 due to a conflict using this values also for
Libgcrypt. Now the values are dumped either with --debug 4 or using
--list-packets along with --verbose.

Because OpenPGP only uses unsigned integers an extra leading zero will
not be printed anymore.

• Signed-off-by: Werner Koch <wk@gnupg.org>