utimens: validate futimens usage

utimens: validate futimens usage

Using gl_futimens(fd,NULL,times) as an implementation for futimens
won't work unless we reliably detect EBADF for out-of-range fd.
Also catches a Linux bug with futimens(AT_FDCWD,NULL) changing ".".
mingw <utime.h> has a bug where utime's second argument is not const.

* lib/utimens.c (gl_futimens): Require valid fd up front, using
fewer syscalls on failure later on.  Avoid compiler warning on
mingw.
* modules/utimens (Depends-on): Add dup2.

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