gluon-mesh-babel: clean up link-local address handling

- 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.