Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

stdbuf: Code cleanup.

* Factor out path-setting code.
* Normalize usage().
* Remove unnecessary switch case.

Sponsored by: Klara, Inc.
Reviewed by: kevans, imp
Differential Revision: https://reviews.freebsd.org/D40799

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

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.

stdbuf(1): Add buffer definition "B" to the usage message

This option has been missing from the usage message ever since the program
was first imported.

Submitted by: shivansh
Reviewed by: asomers
MFC after: 3 weeks
Sponsored by: Google, Inc (GSoC 2017)
Differential Revision: https://reviews.freebsd.org/D11529

Remove stray tabs.

Submitted by: kib
Approved by: kib (mentor)
MFC after: 1 week

Allow commands without any additional arguments, as stated in the
manpage. While here, exit early when there is nothing to do.

PR: 168415
Submitted by: Zhihao Yuan (initial version)
MFC after: 1 week
Approved by: kib (mentor)

Always define LD_32_PRELOAD so it works for 32 bits binaries on
64 bits platforms. Let rtld(1) decide if it needs to honor it
or not.

While here, fix a small bug in error reporting when asprintf(3)
returns an error.

Submitted by: kib
Reviewed by: kib (mentor)
MFC after: 1 week

Use standard getopt(3) error message.

Submitted by: jilles
Approved by: kib (mentor)

Import stdbuf(1) and the shared library it relies on.
This tool changes the default buffering behaviour of standard
stdio streams.

It only works on dynamic binaries. To make it work for static
ones it would require cluttering stdio because there no single
entry point.

PR: 166660
Reviewed by: current@, jhb
Approved by: kib (mentor)
MFC after: 1 week