networkctl: display address labels

```
 ./networkctl label

    Prefix/Prefixlen                          Label
        ::/0                                  1
    fc00::/7                                  5
    fec0::/10                                11
    2002::/16                                 2
    3ffe::/16                                12
 2001:10::/28                                 7
    2001::/32                                 6
::ffff:0.0.0.0/96                                 4
        ::/96                                 3
       ::1/128                                0

```