network: show online state in networkctl status output

In case the link online state is invalid, networkctl will print
"unknown", which is sufficiently neutral. The same goes for the overall
manager online state if there are no managed links, or if
RequiredForOnline=no for all managed links.

Example output:

  $ networkctl status
  ●        State: routable
    Online state: partial
         Address: 172.22.0.130 on wlan0
  ...

  $ networkctl status wlan0
  ● 3: wlan0
               Link File: /lib/systemd/network/99-default.link
            Network File: /etc/systemd/network/50-wlan0.network
                    Type: wlan
                   State: routable (configured)
            Online state: online
  ...