For some reason, the flags used by o.fd.NM.SecretAgent.GetSecrets were
defined as both NMSecretAgentGetSecretsFlags in
libnm{,-glib}/nm-secret-agent.h, and then separately as
NMSettingsGetSecretsFlags in include/nm-settings-flags.h.
(NMSettingsGetSecretsFlags also had an additional internal-use-only
value, but that was added later after the duplication already
existed.)

Fix this by moving NMSecretAgentGetSecretsFlags from libnm to
nm-dbus-interface.h, adding the internal-use-only value to it as well,
updating the core code to use that, and then removing
nm-settings-flags.h.