Deprecate ustat syscall interface

As for sysctl, ustat has been deprecated in favor of {f}statfs.  Also
some newer ports which uses generic interface builds a stub version that
returns ENOSYS.

This patch deprecates ustat interface by removing ustat.h related headers,
adding a compatibility symbol, and avoiding new ports to build and provide
the symbol.

Checked on x86_64-linux-gnu and i686-linux-gnu.  Also checked with a
check-abi on all affected ABIs.

	* NEWS: Add ustat.h deprecation entry.
	* bits/ustat.h: Remove file.
	* misc/sys/ustat.h: Likewise.
	* misc/ustat.h: Likewise.
	* sysdeps/unix/sysv/linux/generic/ustat.c: Likewise.
	* misc/Makefile (headers): Remove ustat.h and sys/ustat.h.
	* misc/ustat.c (__ustat): Rename to __old_ustat and export only in
	compatibility mode.
	* sysdeps/unix/sysv/linux/ustat.c (__ustat): Likewise.
	* sysdeps/unix/sysv/linux/mips/ustat.c: Define DEV_TO_KDEV and use
	generic Linux implementation.