Add more interface change notifications to netlink:
1. Add link-state change notifications by subscribing to `ifnet_link_event`. In the Linux netlink model, link state is reported in 2 places: first is the `IFLA_OPERSTATE`, which stores state per RFC2863. The second is an `IFF_LOWER_UP` interface flag. As many applications rely on the latter, **reserve 1 bit from if_flags, named as IFF_NETLINK_1**. This flag is mapped to `IFF_LOWER_UP` in the netlink headers. This is done to avoid making applications think this flag is actually supported / presented in non-netlink outputs.
2. Add flag change notifications, by hooking into `rt_ifmsg()`. In the netlink model, notification should include the bitmask for the change flags. Update `rt_ifmsg()` to include such bitmask