Merge "Implement unique_fd.ok()"

  explicit unique_fd_impl(int fd) { reset(fd); }

  ~unique_fd_impl() { reset(); }

  unique_fd_impl(const unique_fd_impl&) = delete;

  void operator=(const unique_fd_impl&) = delete;

  unique_fd_impl(unique_fd_impl&& other) noexcept { reset(other.release()); }

  unique_fd_impl& operator=(unique_fd_impl&& s) noexcept {

    int fd = s.fd_;

  // Catch bogus error checks (i.e.: "!fd" instead of "fd != -1").

  bool operator!() const = delete;

  bool ok() const { return get() != -1; }

  int release() __attribute__((warn_unused_result)) {

    tag(fd_, this, nullptr);

    int ret = fd_;

  static auto close(int fd, void*) -> decltype(T::Close(fd), void()) {

    T::Close(fd);

  }

  unique_fd_impl(const unique_fd_impl&);

  void operator=(const unique_fd_impl&);

};

using unique_fd = unique_fd_impl<DefaultCloser>;