futimens: new module

futimens: new module

Provides futimens where it is missing, and rpl_futimens to work
around bugs in older Linux kernels.

* modules/futimens: New file.
* lib/futimens.c (futimens): Likewise.
* m4/futimens.m4 (gl_FUNC_FUTIMENS): Likewise.
* lib/utimens.c (futimens): Avoid recursion into rpl_futimens, so
we can work around Linux bugs.
* m4/sys_stat_h.m4 (gl_SYS_STAT_H_DEFAULTS): Add witnesses.
* modules/sys_stat (Makefile.am): Substitute them.
* lib/sys_stat.in.h (futimens): Declare it.
* MODULES.html.sh (systems lacking POSIX:2008): Mention module.
* doc/posix-functions/futimens.texi (futimens): Likewise.
* modules/futimens-tests: New test.
* tests/test-futimens.c: Likewise.

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