Add new API to allow passing both IPv4 and IPv6 configuration
information from VPN plugins to the backend.

Now instead of a single Ip4Config, a plugin has Config, Ip4Config, and
Ip6Config. "Config" contains information which is neither IPv4 nor
IPv6 specific, and also indicates which of Ip4Config and Ip6Config are
present. Ip4Config now only contains the IPv4-specific bits of
configuration.

There is backward compatibility in both directions: if the daemon is
new and the VPN plugin is old, then NM will notice that the plugin
emitted the Ip4Config signal without having emitted the Config signal
first, and so will assume that it is IPv4-only, and that the generic
bits of configuration have been included with the Ip4Config. If the
daemon is old and the plugin is new, then NMVPNPlugin will copy the
values from the generic config into the IPv4 config as well. (In fact,
NMVPNPlugin *always* does this, because it's harmless, and it's easier
than actually checking the daemon version.)...