Revert "Kernel_socket: do not use the RTF_PROTO2 flag."

This flag is easiest way to filter out babel's route in the
kernel_routes function.

This reverts commit 0845af770694f315398c992c3e9f151f37cbe43a.
BSD: do not attempt atomic route changes.

This is buggy on OS X, at least. As for linux we choose to be safe and
we deactivate atomic route changes on all BSDs.
Makefile: link with librt on Linux only.