networkd: tie links to rtnl rather than udev

networkd: tie links to rtnl rather than udev

This essentially swaps the roles of rtnl and udev in networkd. After this
change libudev is only used for waiting for udev to initialize devices and
to get udev-specific information needed for some [Match] attributes.

This in particular simplifies the code in containers where udev is not really
useful, but also simplifies things and reduces round-trips in the non-container
case.