Add code to NMPppDevice to activate new-style PPPoE connections. This
is a bit tricky because we can't create the link as usual in
create_and_realize(). Instead, we create a device without ifindex and
start pppd in stage2; when pppd reports a new configuration, we rename
the platform link to the correct name and set the ifindex into the
device.

This mechanism is inherently racy, but there is no way to tell pppd to
create an arbitrary interface name.