gpg: Avoid printing false AKL error message.

* g10/getkey.c (get_pubkey_byname): Add special traeatment for default
and skipped-local.

This change avoids error message like

gpg: error retrieving 'foo@example.org' via None: No public key

A 'None' mechanism is something internal.

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