Use binary search to locate an xroute.

Significantly simplifies the xroute logic.

This is safe, since no part of the code ever holds a pointer to an xroute.

This avoids the send_self_retract hack.

This ensures that retractions are sent too.

Also removes stale local routes.

On linux, listen to address and link change events using the netlink socket,
and react to them by dumping all local addresses or all local interfaces,
respectively, to keep babel's view up to date.
This breaks MacOS, as kernel_socket.c needs to grow a kernel_addresses()
function.

Xroutes are now generated dynamically as needed.

This is not compatible with version 0.  Uses 24-byte messages and
doesn't distinguish routes from xroutes.

This avoids inconsistent routing if a neighbour is ingress filtering a
route.

This makes sure we advertise xroutes after they are retracted.  They
will be flushed by the normal GC mechanism.