Allow interface names as scope-id in IPv6 link-local addresses.

Allow interface names as scope-id in IPv6 link-local addresses.

For example, this makes
    forward-zone:
        name: "."
        forward-addr: fe80::20d:b9ff:fe46:c7f4%vio0
        forward-first: yes

work instead of using fe80::20d:b9ff:fe46:c7f4%1 for the forward-addr.

This has been submitted upstream.
sthen@ is fine with carrying this as a local diff.

Requested by & OK kn