The below mentioned commit introduced a regression, that the "wifi"
section of the request type "neighbours" was empty:

    ~# gluon-neighbour-info -d ::1 -r neighbours | ffh_pretty_json
    {
      "wifi": [
      ],
      ...
    }

After this commit, the section (correctly) looks like this:

    root@UFU-FWH-A272-Tresckowstr-GemR-vorne:~# gluon-neighbour-info -d ::1 -r neighbours | ffh_pretty_json
    {
      "wifi": {
        "ca:38:7e:42:5f:21": {
          "neighbours": {
            "fe:9f:4d:01:ea:e1": {
              "noise": -102,
              "inactive": 50,
              "signal": -84
            },
            "fe:df:b9:84:37:51": {
              "noise": -102,
              "inactive": 20,
              "signal": -73
            }
          }
        }
      },
      ...
    }

The issue was due to the fact, that the iteration over the (mesh) wifi interfaces
was broken. The code was assuming, that the section

    config interface 'mesh_radio0'
            option proto 'gluon_mesh'

in /etc/config/network contains an option "ifname", which it does not.
The ifname property is only stored in the corresponding section in
/etc/config/wireless:

    config wifi-iface 'mesh_radio0'
            option ifname 'mesh0'
            option network 'mesh_radio0'
            option mode 'mesh'
            ...

Therefore, we now iterate over wifi-ifaces in /etc/config/wireless, that
have the mode 'mesh' instead. This resolves the issue.

Fixes 0f1fa243

In addition this PR contains:
- split of gluon-respondd provider into multiple source files
- minor additional cleanups in gluon-mesh-babel respondd provider
  (untested, as the babel respondd provider already doesn't compile prior
  to these changes...)