This is part of the preliminaries for upstreaming wg(4).
It is useful to be able to call pr_usrreqs->pru_sockaddr on a struct
socket, so this commit adds a wrapper around it, analogous to the other
types of socket getters.
Original-author: Kyle Evans <kevans@FreeBSD.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>