systemd-networkd and friends have become very chatty.  This commit
is about turning down the noise.  It also removes PreferredLiftime=forever
because it is no longer valid where it used to be, and cannot be used
apparently in the DHCP case.  Seems that the default is now "forever"
anyway, so it's now irrelevant to us.  (Older systemd-networkds would
set the address lifetime to the advertised lease.)

We also only mark an iface with CriticalConnection=yes once that iface
has been chosen as the control net.  We used to just mark them all
in the udev helper so that we didn't have to modify the generated
config after successful detection, but now systemd-networkd complains
about bringing down a searched-but-not-control-net interface if
it is critical.  So, avoid that.

Finally, I added `-q` to our invocation of systemd-networkd-wait-online,
and increased the timeout with which we call it.  Timeout increase is
because we would get spurious event loop disconnect messages without it;
and q to quiet it in other ways.  Ugh.