The advantage is that the API is now the same for IPv4 and IPv6: it's
all nm_dhcp_config_*() and we can (easier) treat the address family
generically.
We still need two distinct GObject types, mainly because of the
glue code for exposing the object on D-Bus as NMDBusObject. Of course,
that could be solved differently, but as it is, it's quite nice.