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.