announce neighbours using alfred/gluon-announce

In Bremen it has been noted that a prefix may lead people to believe it
has to stay there and thus naming their nodes "ffhb-*", which is not
what the community wants (see FreifunkBremen/gluon-site-ffhb#1).

However, an empty prefix lead to the connecting hyphen still being
inserted. This commit thus makes the hyphen part of the configured
prefix and allows the prefix to be missing from the `site.conf`.

Fixes https://github.com/freifunk-gluon/gluon/issues/311

This avoids poking dnsmasq every time a DHCP lease is obtained or a router
advertisement received.

ebtables-filter-multicast: allow ripng

This adds basic peer statistics to statistics.d:

    "mesh_vpn": {
      "muehlentor": {
        "established": 23.8 // seconds
      },
      "huextertor": null,
      "holstentor": null
    }

This adds a new announce.d datum "neighbours" (alfred 160) containing
information about mesh neighbours. It's intended to be an replacement
for batadv-vis.

In addition to the data already provided by batadv-vis it'll also
provide information about direct wifi neighbours.

Unlike batadv-vis, no data about clients is transmitted.

Sample data:

    {
      "wifi": {
        "90:f6:52:82:06:02": {
          "neighbours": {
            "f8:d1:11:2c:a7:d2": {
              "noise": -95,
              "inactive": 0,
              "signal": 0
            },
            "96:f6:52:ff:cd:6f": {
              "noise": -95,
              "inactive": 0,
              "signal": -37
            }
          }
        }
      },
      "batadv": {
        "90:f6:52:82:06:02": {
          "neighbours": {
            "96:f6:52:ff:cd:6f": {
              "lastseen": 2.8500000000000001,
              "tq": 177
            }
          }
        },
        "90:f6:52:82:06:03": {
          "neighbours": {
            "f8:d1:11:2c:a7:d3": {
              "lastseen": 2.3500000000000001,
              "tq": 206
            }
          }
        }
      },
      "node_id": "90f652820602"
    }

Moving the scripts to a common directory not only vastly simplifies the
zzz-gluon-upgrade script, but also allows to define an ordering of such
scripts across packages.