resolvectl: include ifindex when printing link-local DNS server

resolvectl: include ifindex when printing link-local DNS server

Historically, resolvectl status has not included the interface
specification for DNS servers with an IPv6 link-local address, since it
is technically somewhat redundant. But, adding this extra bit of
information makes it easier to copy-and-paste to use elsewhere, etc.

For example, the previous output:

 Link 2 (enp34s0)
     Current Scopes: DNS LLMNR/IPv4 LLMNR/IPv6
          Protocols: +DefaultRoute LLMNR=resolve -mDNS -DNSOverTLS DNSSEC=no/unsupported
 Current DNS Server: fe80::861e:a3ff:feb1:f8e7
        DNS Servers: 192.168.1.12 192.168.1.13 fe80::861e:a3ff:feb1:f8e7
         DNS Domain: lan

now becomes:

 Link 2 (enp34s0)
     Current Scopes: DNS LLMNR/IPv4 LLMNR/IPv6
          Protocols: +DefaultRoute LLMNR=resolve -mDNS -DNSOverTLS DNSSEC=no/unsupported
 Current DNS Server: fe80::861e:a3ff:feb1:f8e7%2
        DNS Servers: 192.168.1.12 192.168.1.13 fe80::861e:a3ff:feb1:f8e7%2
         DNS Domain: lan