libsystemd-networkd: introduce sd-pppoe library

This library negotiates a PPPoE channel. It handles the discovery stage and
leaves the session stage to the kernel. A further PPP library is needed to
actually set up a PPP unit (negotatie LCP, IPCP and do authentication), so in
isolation this is not yet very useful.

The test program has two modes:

  # ./test-pppoe

will create a veth tunnel in a new network namespace, start pppoe-server on one
end and this client library on the other. The pppd server will time out as no
LCP is performed, and the client will then shut down gracefully.

  # ./test-pppoe eth0

will run the client on eth0 (or any other netdev), and requires a PPPoE server
to be reachable on the local link.