networkd: netdev - reduce chance of race when receiving netdev's ifindex

When creating a new link, the kernel will not inform us about the new ifindex
in its ack. We have to listen for newly created devices and deduce the new
ifindex by matching on the ifname.

We used to do this by waiting for a new device from libudev, but that is asking
for trouble, as udev will happily rename the device before handing it to us.
Listen on rtnl instead, the chance of the name being changed before reaching us
is much smaller (if not nil).

Kernel patch in the works to make this unneccessary.