wg: first additions of userspace integration

wg: first additions of userspace integration

This is designed to work with a server that follows this:

  struct sockaddr_un addr = {
      .sun_family = AF_UNIX,
      .sun_path = "/var/run/wireguard/wguserspace0.sock"
  };
  int fd, ret;
  ssize_t len;
  socklen_t socklen;
  struct wgdevice *device;

  fd = socket(AF_UNIX, SOCK_DGRAM, 0);
  if (fd < 0)
      exit(1);
  if (bind(fd, (struct sockaddr *)&addr, sizeof(addr)) < 0)
      exit(1);

  for (;;) {
      /* First we look at how big the next message is, so we know how much to
       * allocate. Note on BSD you can instead use ioctl(fd, FIONREAD, &len). */
      len = recv(fd, NULL, 0, MSG_PEEK | MSG_TRUNC);
      if (len < 0) {
          handle_error();
          continue;
      }
      /* Next we allocate a buffer for the received data. */
      device = NULL;
      if (len) {
          device = malloc(len);
          if (!device) {
              handle_error();
              continue;
          }
      }
      /* Finally we receive the data, storing too the return address. */
      socklen = sizeof(addr);
      len = recvfrom(fd, device, len, 0, (struct sockaddr *)&addr, (socklen_t *)&socklen);
      if (len < 0) {
          handle_error();
          free(device);
          continue;
      }
      if (!len) { /* If len is zero, it's a "get" request, so we send our device back. */
          device = get_current_wireguard_device(&len);
          sendto(fd, device, len, 0, (struct sockaddr *)&addr, socklen);
      } else { /* Otherwise, we just received a wgdevice, so we should "set" and send back the return status. */
          ret = set_current_wireguard_device(device);
          sendto(fd, &ret, sizeof(ret), 0, (struct sockaddr *)&addr, socklen);
          free(device);
      }
  }

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>