Disable mail_crypt when a user has no keys
If a user has no keys, instead of returning a noMatch (N) dict response as we do now, we should probably disable the mail_crypt plugin altogether, or we get this error:
get_public_key(INBOX) failed: mailbox_attribute_get(INBOX, /shared/vendor/vendor.dovecot/pvt/crypt/active) failed: Mailbox attributes not enabled (in reply to end of DATA command))
which is caused by mail_crypt being enabled and the mail_crypt_global_* parameters being unset. To disable a plugin, I think we must instead return a valid response and set mail_plugins to exclude mail_crypt (or so the dovecot docs say).