resolve: Avoid dots in protocol part of interface names when using resolvconf

resolve: Avoid dots in protocol part of interface names when using resolvconf

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