gluon-core: add preserve wifi channel feature

docs: fix typo in authorized keys config example

Ethernet links provide transitive connectivity in all but very unusual
setup, enable mesh_no_rebroadcast to reduce load for devices on links with
many nodes.

Fixes #652

This new feature introduces the new uci section 'gluon-core.wireless' with a preserve_channels option:
 * preserve_channels (boolean)

By setting this option to 1 (true) wifi channels will be preserved during upgrades.

Backport of r47591 and r48427

Should be redundant after 0bd0df6f

Signed-off-by: Felix Kaechele <felix@kaechele.ca>

Change the limit to match our recommendation in the site-example.

We're too close to the release of Gluon 2016.1 to do a full update of our
OpenWrt tree, therefore just make a backport of this.

Fixes #558

Restrict to link-local addresses to prevent amplification attacks from
outside the mesh, or such attacks affecting the outside world.

Fixes #637

Fixes #574

docs: Corrected some typos and strange grammar

If the query was via multicast, the response isn't matched by
--state ESTABLISHED,RELATED

This fixes #619

Instead of starting gluon-respondd from a hotplug handler, add a proper
init script. The new init script has a restart_if_running argument which
is now used by the hotplug handler.