network: move configuration to /etc/systemd/network

This is private configuraiton, so let's not pollute the namespace (and hence make Debian happy :) ).
udev: builtin - rename net_link to net_setup_link

Also add shell completions.
conf-parser: add macro for ENUMV

Parses a whitespace separated list of strings into a vector of enums.
udev: link-config - add proper parsing