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.

configure.ac

@@ -219,7 +219,7 @@ AC_SEARCH_LIBS([socket], [socket])
 AC_SEARCH_LIBS([gethostbyname], [nsl])
 
 if test x$host_is_linux = xyes; then
-	AC_CHECK_FUNCS(pipe2 accept4)
+	AC_CHECK_FUNCS(pipe2 accept4 linkat)
 fi
 
 AC_CHECK_FUNCS(getpwnam_r getpwuid_r)