Avoid commiting configs inbetween to decrease number of flash writes.

Fixes #714.

gluon-announced and gluon-announce are merged to gluon-respondd.

The Gluon-specific functions of the gluon-simple-tc package are moved to
gluon-mesh-vpn-fastd.

Apart from replacing a patch for the former by two patches for latter,
this involved minimal adaptations of the lua scripts in the following
packages:

* gluon-announce
* gluon-announced
* gluon-mesh-batman-adv-core
* gluon-status-page

This will require site.mk to explicitly pull in the gluon-setup-mode and
gluon-config-mode-core packages.

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 patch allows fastd's enabled flag's default value to be set from
site.conf.

Previously, the config-mode was responsible for generating the fastd
secret. This patch sets the default secret to "generate" causing a
secret to be generated on its first use (e.g. show_key or start).

This also changes the info page (in Expert Mode) to show "n/a" when the
public key is not yet available.

The support is still very limited (IPv6-only DNS on WAN doesn't work yet), and
we now need fastd running as root, but apart from that, is should work.

All announce.d scripts have been moved to /lib/gluon/announce/announce.d
The script /lib/gluon/announce/announce.lua will collect all information
and output json.

This also fixes the type of a few fields which were strings instead of numbers.

Better configure these properly.