MFC: r310712

- Use correct offsets into the keys set array. As the elements of this
zero-length array are dynamically sized at run-time based on the use
of hints, compilers can't be expected to figure out these offsets on
their own. [1]
- Fix incorrect comparison in cmp_nans(). [2]

PR: 204571 [1], 202301 [2]
Submitted by: David Binderman [2]

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

- Update Oleg Moskalenko's email address

Requested by: Oleg Moskalenko <mom040267@gmail.com>

- Portability changes for ARM
- Allow larger sort memory on 64-bit platforms

Submitted by: Oleg Moskalenko <oleg.moskalenko@citrix.com>

- Remove the UNUSED_ARG macro and use __unused in argument lists

Reviewed by: dim
MFC after: 3 days

- Eliminate initializations if global variables. Compilers are not
required to optimize these so it may result in larger binary size.

Pointed out by: kib

Add a BSD-licensed sort rewrite that was started by me and later completed
with the major functionality and optimizations by Oleg Moskalenko.
It is compatible with the latest version of POSIX and the current GNU sort
version that we have in base. Beside this, it implements all the
functionality introduced in later versions of GNU sort. For now, it will
be installed as "bsdsort", keeping GNU sort as the default sort
implementation.