Fixes #1959

Provide the number of OWE stations in addition to the number of all
connected wireless clients.

In addition this PR contains:
- split of gluon-respondd provider into multiple source files
- minor additional cleanups in gluon-mesh-babel respondd provider
  (untested, as the babel respondd provider already doesn't compile prior
  to these changes...)

Fixes #1701

This commit adds information about:
- how cpu time is spent since boot in jiffies (1/100*sek) (cpu)
    - the value is summed for all cores, so in 10 seconds the
      summed values will increase by 4000, if the cpu has
      4 cores
- context switches since boot (ctxt)
- interrupt counters since boot (intr, softirq)
- forks since boot (processes)

    { "stat": {
       "cpu": {
         "user": 219403,
         "nice": 1714,
         "system": 75159,
         "idle": 2727739,
         "iowait": 2943,
         "irq": 0,
         "softirq": 571
       },
       "intr": 8426340,
       "ctxt": 50992590,
       "processes": 10549,
       "softirq": 5161884
    } }

Our provider modules are prefixed with gluon- now, and the log message
about a missing provider dir was confusing users.

[Matthias Schiffer: rebase]

 * 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

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.

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

Mesh-wide multicast requests are answered with a random delay of up to 10s.

Because we unconditionally appended `-i br-client` to the command line of
respondd, it wasn't restarted when br-client changed state. Now, we use a
jsonfilter expression on the network.interface dump data, similar to how the
other interface names are generated, and only add the interface to the argument
list if it is up.

Fixes #414

Most doubles that are delivered via respondd have limited input
precision, but are converted with up to 17 digits of precision. That can
cause ugly blowups like 0.2800000000000001 in the output, which is
avoided by specifying better format strings (like "%.2f" in most cases).

Avoid commiting configs inbetween to decrease number of flash writes.

Fixes #714.

Should be redundant after 0bd0df6f

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

Fixes #637

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.

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