We will need access to the serialization flags from within the synth_func().

That will be for WireGuard's peers. Peers are a list of complex, structured
elements, and some fields (the peer's preshared-key) are secret and
others are not. So when serializing the peers, we need to know whether
to include secrets or not.

Instead of letting _nm_setting_to_dbus() check the flags, pass them
down.

While at it, don't pass the property_name argument. Instead, pass the
entire meta-data information we have. Most synth functions don't care
about the property or the name either way. But we should not pre-filter
information that we have at hand. Just pass it to the synth function.
If the synth function would be public API, that would be a reason to be
careful about what we pass. But it isn't and it only has one caller.
So passing it along is fine. Also, do it now when adding the flags
argument, as we touch all synth implementations anyway.