Augment the KPI with new if_getaddr(), that returns struct sockaddr_dl
pointer for the interface link level address.

The existing if_getlladdr(), which calculates pointer to the actual
address withing the sockaddr_dl, is already widely used and its name
reminds the old LLADDR() macro, which is helpful to BSD old timers. It
isn't helpful for newcomers, though. But the wide use of if_getlladdr()
really prevents a sweeping change just for better naming purposes. While
most consumers are happy with if_getlladdr(), some new consumers want full
access to sockaddr_dl, that the new function provides.