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-expose-ports.c

@@ -194,7 +194,7 @@ int expose_port_send_rtnl(int send_fd) {
 
         /* Store away the fd in the socket, so that it stays open as
          * long as we run the child */
-        r = send_one_fd(send_fd, fd);
+        r = send_one_fd(send_fd, fd, 0);
         if (r < 0)
                 return log_error_errno(r, "Failed to send netlink fd: %m");
 
@@ -214,7 +214,7 @@ int expose_port_watch_rtnl(
         assert(recv_fd >= 0);
         assert(ret);
 
-        fd = receive_one_fd(recv_fd);
+        fd = receive_one_fd(recv_fd, 0);
         if (fd < 0)
                 return log_error_errno(fd, "Failed to recv netlink fd: %m");
 

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)