Also make babel match batman-adv and only emit the wireless/tunnel/other
fields when they are non-empty.

Fixes: #1783

- Rename obtain_if_addr() to get_linklocal_address()
- Pass buffer of size INET6_ADDRSTRLEN instead of the oversized NI_MAXHOST
- Check if an address is link-local before converting to a string
- Replace an incorrect use of strncmp() with strcmp()
- Return status to caller
- Streamline control flow

While we're at it, the function handle_neighbour(), which is one of the
callers of get_linklocal_address() is slightly cleaned up as well.

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...)

Adjust for changed libbabelhelper API.

specify command 'dump' for now until babeld supports filtering

increase size of path by 1 to be able to fit the full path including trailing \0 byte into array

gluon-mesh-babel conigures babeld and thus the mesh. This mesh is only usable
with routes to the clients which are discovered by l3roamd.

log-file /dev/stderr is broken for babeld as it eats log messages for debug log.
This commit gets rid of the option. This allows -d N to be used as babeld command
line option.

gluon-core, gluon-mesh-babel: move lua functions that generate ll-addresses from a mac into gluon-core

gluon-mesh-babel: babel-respond: stop relying on mac addresses and instead obtain ll-address from nic

Rename to respondd.c / respondd.so, gluon.mk expects these names. This way
we can remove the install code. The installed filename is changed to
gluon-mesh-babel.so, bringing it in line with out common naming scheme.