The networkd.conf file controls a variety of interesting settings
which don't seem to be configurable at the moment, including
adding names to route tables (for networkd only, although this commit
also exports them into iproute2 for convenience's sake), and
the speed metering functionality built into networkd.
Importantly, however, this also allows disabling the systemd
functionality where it likes to delete all the routes and routing rules
that haven't been configured through networkd whenever something causes
it to perform a reconfiguration.
f47caf769b