resolved: fix comments in resolve.conf for search domain overflows (#3422)

resolved: fix comments in resolve.conf for search domain overflows (#3422)

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