Deprecate external use of libio.h and _G_config.h.

Deprecate external use of libio.h and _G_config.h.

libio.h was originally the header for a set of supported GNU
extensions, but they have not been maintained as such in many years,
they are now standing in the way of improvements to stdio, and we
don't think there are any remaining external users.  _G_config.h was
never intended for public use, but predates the bits convention.
Move both of these headers into the bits directory and provide stubs
at top level which issue deprecation warnings.

The contents of (bits/)libio.h and (bits/)_G_config.h are still
exposed to external software via stdio.h; changing that requires more
complex surgery than I have time to attempt right now.

	* libio/libio.h, libio/_G_config.h: New stub headers which issue a
	deprecation warning and then include <bits/libio.h>, <bits/_G_config.h>
	respectively.
	* libio/libio.h: Rename the original version of this file to
	libio/bits/libio.h.  Error out if not included by stdio.h or the
	stub libio.h.
	* include/libio.h: Move to include/bits.  Forward to libio/bits/libio.h.
	* sysdeps/generic/_G_config.h: Move to top-level bits/.  Error out
	if not included by bits/libio.h or the stub _G_config.h.
	* sysdeps/unix/sysv/linux/_G_config.h: Move to
	sysdeps/unix/sysv/linux/bits.  Error out if not included by
	bits/libio.h or the stub _G_config.h.
	* libio/stdio.h: Include bits/libio.h, not libio.h.
	* libio/Makefile: Install bits/libio.h and bits/_G_config.h as
	well as libio.h and _G_config.h.

	* csu/init.c, libio/fmemopen.c, libio/iolibio.h, libio/oldfmemopen.c
	* libio/strfile.h, stdio-common/vfscanf.c
	* sysdeps/pthread/flockfile.c, sysdeps/pthread/funlockfile.c
	Include stdio.h, not _G_config.h nor libio.h.
	* libio/iofgetpos.c: Also rename fgetpos64 out of the way.
	* libio/iofsetpos.c: Also rename fsetpos64 out of the way.

	* scripts/check-installed-headers.sh: Skip libio.h and _G_config.h.