diff: Integrate libdiff from OpenBSD GoT.

This adds support for two new diff algorithms, Myers diff and Patience
diff.

These algorithms perform a different form of search compared to the
classic Stone algorithm and support escapes when worst case scenarios
are encountered.

Add the -A flag to allow selection of the algorithm, but default to
using the new Myers diff implementation.

The libdiff implementation currently only supports a subset of input and
output options supported by diff. When these options are used, but the
algorithm is not selected, automatically fallback to the classic Stone
algorithm until support for these modes can be added.

Based on work originally done by thj@ with contributions from kevans@.

Sponsored by: Klara, Inc.
Reviewed by: thj
Differential Revision: https://reviews.freebsd.org/D44302

Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

Fix indentation in usr.bin/diff/pr.c

In commit 6fa5bf0832ef the pr(1) related code in diff was moved around,
but some part of the indentation was messed up, and one line was
duplicated. Remove the duplicated line, and fix up the indentation.

Reviewed by: bapt
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D34398

Fix memory leak

CID: 1393352

Correctly test return value of pipe(2)

CID: 1393351

Isolate the pr(1) related code in its own source files

This keeps diffreg.c closer to what it is supposed to do:
diffing regular files.

It also allows my code to get a proper license