The resolvconf implementation provided by systemd via resolvectl strips
everything after the interface name, so each additional server that's
installed replaces the previous one. And even for other resolvconf
implementations installing them individually doesn't seem necessary as
we track and refcount them anyway.
Closesstrongswan/strongswan#1353
Newer releases of systemd contain a change that removes not the part
after the first dot but the part after the last when determining the
interface name (apparently some interface names actually contain a dot).
This changes the default prefix to only contain one dot and avoids the
dots added by IPv4 addresses to create a unique interface/protocol for
each DNS server (it also replaces the `:` in IPv6 addresses with
something that might cause less conflicts).
References strongswan/strongswan#1353
Tobias Brunner