Following the changes to expose the "routing" and "search" domain concepts in networkd, actually make resolved use
them. It will now use routing domains exclusively for making DNS routing decisions, and use search domains additionally
for extending single-label names.
Not only report whether the server actually supports DNSSEC, but also first check whether DNSSEC is actually enabled
for it in our local configuration.
Also, export a per-link DNSSECSupported property in addition to the existing manager-wide property.
So far, we exposed SetLinkXYZ() on the Manager interface, to set a couple of link properties. This adds similar calls
SetXYZ() on the Link interface, and makes sure the former is little more than a shortcut to the latter.
SetLinkXYZ() has the benefit of not requiring a GetLink() round trip for setting these properties, while the method
actually belongs to the Link objects, and this change corrects that.
Lennart Poettering