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)

Fixes: 3b145a33b748935b17a83c85e451406698e6cec3

https://bugzilla.gnome.org/show_bug.cgi?id=757287

Tunnelled VPNs can need that so that correct source address is used for tunnel
routes.

It's an IPv4 address.

The configuration is going to be applied on the parent interface.

If we didn't do this, an attempt to add a route would result in an assertion
failure:

  nm-ip4-config.c:1475:nm_ip4_config_add_route: assertion failed: (priv->ifindex)

Maybe it's not mounted in the first place. We could be running in a chroot
without /sys mounted, etc.

Also, the error message was wrong.

gcc 5.1.1 wrongly warns about uninitialized variable @r2 when compiling
with -Og. Refactor the code to avoid the warnings.

Related: https://github.com/systemd/systemd/pull/1735