cli: add command for displaying LLDP neighbors

cli: add command for displaying LLDP neighbors

The list of LLDP neighbors is available through the D-Bus interface
and libnm already provides functions to retrieve it; make the list
available through nmcli as well. Sample output:

  $ nmcli device lldp
  NEIGHBOR[0].DEVICE:                     eth0
  NEIGHBOR[0].CHASSIS-ID:                 00:13:21:58:CA:42
  NEIGHBOR[0].PORT-ID:                    1
  NEIGHBOR[0].PORT-DESCRIPTION:           1
  NEIGHBOR[0].SYSTEM-NAME:                ProCurve Switch 2600-8-PWR
  NEIGHBOR[0].SYSTEM-DESCRIPTION:         ProCurve J8762A Switch 2600-8-PWR, revision H.08.89
  NEIGHBOR[0].SYSTEM-CAPABILITIES:        20 (mac-bridge,router)
  NEIGHBOR[1].DEVICE:                     eth2
  NEIGHBOR[1].CHASSIS-ID:                 00:01:30:F8:AD:A2
  NEIGHBOR[1].PORT-ID:                    1/1
  NEIGHBOR[1].PORT-DESCRIPTION:           Summit300-48-Port 1001
  NEIGHBOR[1].SYSTEM-NAME:                Summit300-48
  NEIGHBOR[1].SYSTEM-DESCRIPTION:         Summit300-48 - Version 7.4e.1 (Build 5)
  NEIGHBOR[1].SYSTEM-CAPABILITIES:        20 (mac-bridge,router)

https://bugzilla.gnome.org/show_bug.cgi?id=757307