Add renameat2 function [BZ #17662]

The implementation falls back to renameat if renameat2 is not available
in the kernel (or in the kernel headers) and the flags argument is zero.
Without kernel support, a non-zero argument returns EINVAL, not ENOSYS.
This mirrors what the kernel does for invalid renameat2 flags.

sysdeps/unix/sysv/linux/renameat2.c

@@ -0,0 +1,44 @@
+/* Linux implementation for renameat2 function.
+   Copyright (C) 2018 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 <errno.h>
+#include <stdio.h>
+#include <sysdep.h>
+
+int
+renameat2 (int oldfd, const char *old, int newfd, const char *new,
+           unsigned int flags)
+{
+#if !defined (__NR_renameat) || defined (__ASSUME_RENAMEAT2)
+  return INLINE_SYSCALL_CALL (renameat2, oldfd, old, newfd, new, flags);
+#else
+  if (flags == 0)
+    return __renameat (oldfd, old, newfd, new);
+# ifdef __NR_renameat2
+  /* For non-zero flags, try the renameat2 system call.  */
+  int ret = INLINE_SYSCALL_CALL (renameat2, oldfd, old, newfd, new, flags);
+  if (ret != -1 || errno != ENOSYS)
+    /* Preserve non-error/non-ENOSYS return values.  */
+    return ret;
+# endif
+  /* No kernel (header) support for renameat2.  All flags are
+     unknown.  */
+  __set_errno (EINVAL);
+  return -1;
+#endif
+}