Sometimes there seemed to be a race where systemd-networkd would start
trying to find .network config files before the emulab control net udev
rules (the shell script the rule launches, rather) could actually write
them.  This race would sometimes manifest, but mostly not.

By default, systemd-networkd does not wait for udev to finish
processing its device events.  This seems odd, but I guess it is not
expecting dynamic configuration in /run/systemd/network like we use
(although its docs explicitly allow for this case).  Oh well.  `udevadm
settle` is supposed to indicate that udev is done processing all device
events; but it must not mean that it is finished processing all the
rules!  So not only do we wait for it to settle; we wait for the
appearance of at least one .network file (that we wrote, hopefully) in
/run/systemd/network .  No one else would ever write them there,
clearly.