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.