core: implement activation of PPP devices
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.
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