networkctl: Add support to display min and max MTU

networkctl: Add support to display min and max MTU

(networkctl)⚡ % ./networkctl status enp0s31f6                                                                                                                          ~/tt/networkctl/build
● 4: enp0s31f6
       Link File: /usr/lib/systemd/network/99-default.link
    Network File: n/a
            Type: ether
           State: n/a (unmanaged)
            Path: pci-0000:00:1f.6
          Driver: e1000e
          Vendor: Intel Corporation
           Model: Ethernet Connection (2) I219-LM
      HW Address: 8c:16:45:6c:83:b9 (LCFC(HeFei) Electronics Technology co., ltd)
             MTU: 1500
     Minimum MTU: 68
     Maximum MTU: 9000