fs/io/FileOutputStream: use O_TMPFILE if available

The Linux feature allows writing new files to an invisible file, and
then replace the old file.  This preserves the old file if we get
interrupted by some event.

src/fs/io/FileOutputStream.hxx

@@ -42,6 +42,14 @@ class FileOutputStream final : public OutputStream {
 	int fd;
 #endif
 
+#ifdef HAVE_LINKAT
+	/**
+	 * Was O_TMPFILE used?  If yes, then linkat() must be used to
+	 * create a link to this file.
+	 */
+	bool is_tmpfile;
+#endif
+
 public:
 	FileOutputStream(Path _path, Error &error);