usr.bin: Remove ancient SCCS tags.

Remove ancient SCCS tags from the tree, automated scripting, with two
minor fixup to keep things compiling. All the common forms in the tree
were removed with a perl script.

Sponsored by: Netflix

Remove $FreeBSD$: one-line nroff pattern

Remove /^\.\\"\s*\$FreeBSD\$$\n/

diff3: Add man page description for version and help

Reviewed by: Pau Amma
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D35325

diff3: Add support for -m

diff3 in -m mode generates a complete file with changes bracketed with
conflict markers. This adds support for diff3 to generate version
control style three way merge output.

The output format was inferred from looking at the gnu diff3 output on a
selection of test files as a specification of what diff3 -m should
output is not available. It is likely there are cases where the -m
output differs from other tools and I am happy to update diff3 to
address these.

Discussed with: pstef, kevans
Sponsored by: Klara, Inc.

diff3: Add support for -A

Diff3 in -A mode generates an ed script to show how the 3 files and
brackets changes that conflict. The ed script generated should when
applied leave familiar merge conflict markers in a patched file.

Diff3 output is not documented, this feature has been arrived at by
comparing bsd diff3 output to gnu diff3 output until they were made to
agree. There are likely to still be differences between these formats.

The gnu diff3 guide is actually quite good at explaining how diff3
output should appear, but it doesn't cover every form of output from
diff3.

https://www.gnu.org/software/diffutils/manual/diffutils.html#Comparing-Three-Files

Discussed with: pstef, kevans
Sponsored by: Klara, Inc.

diff3: document and test -T

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D31650

diff3: implement --strip-trailing-cr

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D31626

diff3.1: update manual page to match code

Update the diff3 manpage to reflect the fact the version in freebsd does
not use temporary files nor uses a /usr/libexec/diff3prog

Import working progress BSD diff3

import bsd diff3 from OpenBSD.
Differences with OpenBSD:
- lots of warning fixed
- no shell wrapper with diff3 actually living in libexec
- capsicumized

Keep it disconnected as it is not yet good enough to replace GNU diff

The motivation to import it now it to allow other people to jump in and also to
have an open development on it

Obtained from: OpenBSD