The connection.mdns setting is a per-connection setting,
so one might expect that one activated device can only have
one MDNS setting at a time.

However, with certain VPN plugins (those that don't have their
own IP interface, like libreswan), the VPN configuration is merged
into the configuration of the device. So, in this case, there
might be multiple settings for one device that must be merged.

We already have a mechanism for that. It's NMIP4Config. Let NMIP4Config
track this piece of information. Although, stricitly speaking this
is not tied to IPv4, the alternative would be to introduce a new
object to track such data, which would be a tremendous effort
and more complicated then this.

Luckily, NMDnsManager and NMDnsPlugin are already equipped to
handle multiple NMIPConfig instances per device (IPv4 vs. IPv6,
and Device vs. VPN).

Also make "connection.mdns" configurable via global defaults in
NetworkManager.conf.