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.

When adding unreachable routes and setting the RTNH_F_ONLINK flag, a
device is required to be specified. In Linux kernel 4.16 support for
this flag was added. Until now it was ignored.
If RTNH_F_ONLINK is specified while the device is missing, newer kernels
will respond with No such device.
The result is:
* spam in the log file
* missing routes for both ipv4 and ipv6