When a Gluon node is used to connect to an uplink router/DHCP server (for
example in deployments without VPN tunnels), the gw_mode must be set to
server; this should be preserved on upgrades.

Fixes #1196

[Matthias Schiffer: move to proto_gluon_bat0_setup() and default to BATMAN_IV]

The next-node MAC address doesn't need to be unique in different
communities, so we can as well add a default value.

Reverts d5829d87 ("gluon-mesh-batman-adv-core: disable bridge port
learning on bat0").

Fixes #1121

We now create bat0 and primary0 independently of the lower mesh interfaces,
making the whole setup a lot more robust. In particular:

- we can't accidentially destroy primary0 because of concurrent setup and
  teardown runs of different interfaces
- bat0 will always exist, even when no mesh interfaces are up (e.g. no link
  on wired mesh)
- interfaces going down and up again will never tear down the whole of
  batman-adv
- we can enable and disable bat0 independently of the lower interface
  states

For simplicity, we don't use different MTUs for compat 14 and 15 anymore,
there's no harm in using 1532 for batman-adv-legacy as well.

Also move it to gluon-mesh-batman-adv, as mesh_on_wan is the only feature that
needs a unique MAC address on the WAN interface.

The now empty gluon-firewall is removed.

This will make a node announce all MACs of its interfaces participating
in the batman-adv mesh. This enables other nodes to associate the
announced object with both the data reported by batadv-vis as well as a
simple list of neighbours as output by `iw dev $IFACE station dump`.

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.

For security reasons we should not accept incoming packets per default
and instead allow specific services on specific interfaces.

gluon-mesh-batman-adv should not be used without firewall to prevent forwarding
of packets from the mesh to the wan interface.

For some reason, the firewall package won't work at all without
kmod-ipt-nathelper.