Fixes #1043

In preparation for Babel support.

Remove obsolete x86-xen_domu definition, it has been merged into
x86-generic.

uradvd will tolerate devices appearing and disappearing during runtime.

Also remove our own no_rebroadcast patch, as batman-adv v2016.5 now has a
more sophisticated rebroadcast suppression that should work automatically
in the most relevant cases.

These values have been in use in gluon-core for some time, the validators
need to be moved as well.

macvlan interfaces never directly exchange traffic with the underlying
interface, but only with other hosts behind the interface. In consequence,
router advertisements from the uradvd running on br-client could never
reach local-node, preventing it from getting an IPv6 address without RAs
from an external radvd. Fix this be replacing the macvlan interface with
a veth pair (with the peer interface in br-client).

As a side effect, this saves about 5KB of flash, as the veth module is
simpler than macvlan.

When preparing the migration from macvlan to veth for local-node, MAC
address conflicts occurred as some ports of br-client had the same address
as local-node. Reverting the roles of both interfaces fixes this.

By default, br-client is left as an interface without addresses and
firewall rules that drop everything, so the bridge is used to connect its
ports only. gluon-mesh-batman-adv-core changes this to the usual set
of addresses and firewall rules.

Prepare reverting the roles of br-client and local-node to their state
before 8c4403ba.

Also simplify the local_node.peerdns setting.

Nothing outside the config mode depends on luci-base anymore.

We also make use of the boolean support of simple-uci to make scripts
clearer.

The maintenance overhead doesn't justify keeping this old stuff anymore.

Rework init script to additionally join a ff05::... multicast group on the mesh-wide interface.

Add missing trailing `,` in simple_tc section and change the order of ingress and egress to default format used by Gluon.