Fixes #1311

Fixes #1308

The 'auto' attribute still allows enabling the interface using ifup, which
is not intended when wired mesh is disabled.

As core defines basic rules for this zone, it makes sense to create it
there.

Based-on-patch-by: Christof Schulze <christof.schulze@gmx.net>

OUTPUT is always accepted, no need to allow ICMPv6 explicitly.

Wrap long lines.

 * gluon-core, gluon-client-bridge: introduce new firewall zone: local_client
 * gluon-core: put clients in local_client zone, introduce drop-zone,
   set dns-rules and zones
 * gluon-respondd: allow respondd on mesh
 * gluon-status-page-api: allow http input on mesh and client

Some files have received some additional refactoring.

In particular, this affects users of gluon.util.iterate_radios.

The new gluon.site lua library will eventually replace gluon.site_config
(which is hereby deprecated, but will continue to be supported for a
while).

The new gluon.site library will wrap all values to allow traversing
non-existing tables without errors.

    site = require 'gluon.site'
    c = site.a.b.c -- doesn't fail even if a or a.b don't exist

The wrapped values must be unwrapped using call syntax:

    site_name = site.site_name()

Using the call syntax on a non-existing value will return nil. An
alternative default value may be passed instead:

    mac = site.next_node.mac('16:41:95:40:f7:dc')

uci:set_list() will delete the list when nil is passed, so there is no need
to differentiate between the cases.

Based-on-patch-by: Christof Schulze <christof.schulze@gmx.net>

While we use the hexadecimal representation as a hash input for simplicity,
it should not be interpreted as case-sensitive.

The new proto will simplify the switch to VXLAN encapsulation.

LEDE recently disabled multicast snooping by default:

https://git.lede-project.org/?p=project/netifd.git;a=commitdiff;h=52541140f8138e31958cdc3d7e42a4029fa6bbc9



Reenable it for Gluon as there have been no confirmed issues for
LEDE and no negative reports concerning Gluon v2016.2.x so far.

Closes #1025.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>

In addition, transitive and fixed_mtu now default to 0 instead of an empty string.

gluon-core: gluon.util: pass UCI cursor as argument to iterate_radios() instead of using local context