openat-safer: new module

openat-safer: new module

* modules/openat-safer: New file.
* lib/openat-safer.c: Likewise.
* m4/fcntl-safer.m4 (gl_OPENAT_SAFER): New macro.
* lib/fcntl-safer.h (openat_safer): Declare.
* lib/fcntl--.h (openat): Override.
* MODULES.html.sh (File descriptor based I/O): Mention it.
* lib/openat.h: Add double-inclusion guards.
* lib/openat.c (includes): Only include "fcntl-safer.h", not
"fcntl--.h", so we can implement openat.
* modules/openat-safer-tests: New test.
* tests/test-openat-safer.c: New file.

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