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
    }

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.

This also fixes a long-broken legacy upgrade script which would leave the
sysconfig variable lan_ifname unset and the LAN ports unusable.