renameat: new module

renameat: new module

Passes on Linux and cygwin 1.7 native renameat, and on systems
lacking renameat, but fails on Solaris 9 and 10 for now.

* modules/renameat: New file.
* lib/renameat.c (renameat): Likewise.
* m4/renameat.m4 (gl_FUNC_RENAMEAT): Likewise.
* m4/stdio_h.m4 (gl_STDIO_H_DEFAULTS): Add witnesses.
* modules/stdio (Makefile.am): Substitute them.
* lib/stdio.in.h (renameat): Declare it.
* MODULES.html.sh (systems lacking POSIX:2008): Mention module.
* doc/posix-functions/renameat.texi (renameat): Likewise.
* modules/renameat-tests: New test.
* tests/test-renameat.c: Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>