networkd : verify dns ip address when parsing configuration (#4492)

networkd : verify dns ip address when parsing configuration (#4492)

Invalid IP addresses would be passed through as-is:
$ networkctl status wlp3s0:
● 2: wlp3s0
       Link File: /usr/lib/systemd/network/99-default.link
    Network File: /etc/systemd/network/wlp3s0.network
            Type: wlan
           State: routable (configured)
            Path: pci-0000:03:00.0
          Driver: iwlwifi
          Vendor: Intel Corporation
           Model: Centrino Advanced-N 6205 [Taylor Peak] (Centrino Advanced-N 6205 AGN)
      HW Address: XXXXXXXXXX (Intel Corporate)
         Address: 192.168.2.103
                  XXXXXXXXXXX
         Gateway: 192.168.2.1 (Arcadyan Technology Corporation)
             DNS: 127.0.0.5553

Instead verify that DNS= has a valid list of addresses when parsing configuration.

Fixes #4462.