copy_file_range: New function to copy file data

The semantics are based on the Linux system call, but a very close
emulation in user space is provided.

support/xftruncate.c

@@ -0,0 +1,27 @@
+/* ftruncate with error checking.
+   Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/check.h>
+#include <support/xunistd.h>
+
+void
+xftruncate (int fd, long long length)
+{
+  if (ftruncate64 (fd, length) != 0)
+    FAIL_EXIT1 ("ftruncate64 (%d, %lld): %m", fd, length);
+}