Write comments about "too many search domains" and "Total length of all search
domains is too long" just once. Also put it on a separate line, as
resolv.conf(5) only specifies comments in a line by themselves.
This is ugly to do if write_resolv_conf_search() gets called once for every
search domain. So change it to receive the complete OrderedSet instead and do
the iteration by itself.
Add test cases to networkd-test.py.
https://launchpad.net/bugs/1588229
This uses temporary configuration in /run and temporary veth devices, and does
not write anything on disk or change any system configuration; but it assumes
(and checks at the beginning) that networkd is not currently running.
This can be run on a normal installation, in QEMU, nspawn, or LXC.
As this requires root privileges, this is not integrated into "make check".