resolved: add a new DnsScopeOrigin enum, to delcare the "origin" of a DnsScope explicitly

resolved: add a new DnsScopeOrigin enum, to delcare the "origin" of a DnsScope explicitly

This new enum field is supposed to indicate why a DnsScope came to be.
For now it distinguishes two origins: the "global" one (which is what is
configured in resolved.conf) and "link" ones (which are synthesized for
each link).

The field as is is pretty redundant, the same information can be
determined from whether the .link field is set or not.

This is pretty much just preparation for later commits that add
statically configured additional DnsScopes whose origin shall be encoded
with this.