tree-wide: port more code to use send_one_fd() and receive_one_fd()

Also, make it slightly more powerful, by accepting a flags argument, and
make it safe for handling if more than one cmsg attribute happens to be
attached.

src/nspawn/nspawn.c

@@ -1291,7 +1291,7 @@ static int setup_kmsg(const char *dest, int kmsg_socket) {
 
         /* Store away the fd in the socket, so that it stays open as
          * long as we run the child */
-        r = send_one_fd(kmsg_socket, fd);
+        r = send_one_fd(kmsg_socket, fd, 0);
         safe_close(fd);
 
         if (r < 0)