Only set rp_filter to 0 if it contains a different value when
initializing new interfaces. This is helpful to run babeld inside docker
without --privileged

Thanks to Dave Taht.

This allows walking the two lists in parallel.

Use binary search to locate an xroute.

Reported by cppcheck.

(portability) '(void*)((char*)current_attribute)' is of type
'void *'. When using void pointers in calculations, the behaviour is
undefined.

We can no longer call send_multihop_request with a NULL argument.

This removes duplication between unicast and multicast variants of message
formatting.

This is no longer useful and complicates the code.

Thanks to Christof Schulze for noticing the problem.