Disable the keyring if the backend is not good enough
The client should disable the keyring when the best backend is the EncryptedKeyring (or the PlaintextKeyring). This is mainly for Linux, where it's not easy to use the desktop provided keyring because of our bundling.
(from redmine: created on 2013-06-21, closed on 2013-06-30, relates #2638 (closed), relates #2961 (closed))