At various places we only want to serialize agent-owned secrets. Without this
flag, we need to clone the setting first, then drop the secrets, then serialize
to D-Bus. Add a serialization flag to avoid that.

The name ("with") and the meaning of the flag is chosen in a way, that
there could be multiple such flags (NM_CONNECTION_SERIALIZE_WITH_SECRETS_NOT_REQUIRED),
and specifying at least one of them, would have the meaning to whitelist
flags of this kind. Specifying non of these "with" flags would have the
meaning of specifying *all*. Currently there is only one kind, so the name
and meaning is slightly counter intuitive.