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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

sort: cleanup now unused structutre and prototypes

sort: remove unused function

usr.bin/sort: Avoid UBSan errors

UBSan complains about out-of-bounds accesses for zero-length arrays. To
avoid this we can use flexible array members. However, the C standard does
not allow for structures that only contain flexible array members, so we
move the length parameters into that structure too.

Split out from D28233.

Reviewed By: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31009

sort: Cache value of MB_CUR_MAX

Every usage of MB_CUR_MAX results in a call to __mb_cur_max. This is
inefficient and redundant. Caching the value of MB_CUR_MAX in a global
variable removes these calls and speeds up the runtime of sort. For
numeric sorting, runtime is almost halved in some tests.

PR: 255551
PR: 255840
Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30170

various: general adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using misidentified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

No functional change intended.

Various style(9) fixes and typos in grep, sort and patch.

MFC after: 3 days

- 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>

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.