netlink: Expose method to get Netlink pid of a socket.

In the future, the kernel will use unicast messages instead of
multicast to send upcalls.  As a result, we need to be able to
tell it where to direct the traffic.  This adds a function to expose
the Netlink pid of a socket so it can be included in messages to the
kernel.

lib/netlink-socket.c

@@ -667,6 +667,13 @@ nl_sock_wait(const struct nl_sock *sock, short int events)
 {
     poll_fd_wait(sock->fd, events);
 }
+
+/* Returns the PID associated with this socket. */
+uint32_t
+nl_sock_pid(const struct nl_sock *sock)
+{
+    return sock->pid;
+}
 
 /* Miscellaneous.  */