fcntl-h, stdio, sys_ioctl: fix declarations

fcntl-h, stdio, sys_ioctl: fix declarations

We cannot use a function-like macro to provide a link-time warning
of a variadic function during GNULIB_POSIXCHECK; at least, not
without assuming C99 variadic macros.  We can, however,
use an object-like macro (as was already done for printf).

On the other hand, this patch is only a stop-gap measure to fix
an obvious bug; a more complete patch that switches from a
link-time warning (specific to GNU ld, gcc, and ELF image) to
a compile-time attribute is in the works.

* lib/stdio.in.h (dprintf): Use of link warning on a variadic
function must not take arguments.
* lib/sys_ioctl.in.h (ioctl): Likewise.
* lib/fcntl.in.h (openat): Likewise.  Declare extern.
(open): Add a link warning.

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