rtnl: don't ignore broadcast messages
rtnl: add callback support

sd_rtnl_add_match allows you to add a callback function for when given types of
messages are received.
rtnl: add link_get_flags
networkd: add link-sense and simplify state-machine a bit

This listens to rtnetlink for changes to IFF_UP and IFF_LOWER_UP (link sense). The latter
is simply logged at the moment, but will be useful once we add dhcp support.
TODO: add networkd entries