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/

asprintf returns -1, not an arbitrary value < 0. Also upon error the
(very sloppy specification) leaves an undefined value in *ret, so it is
wrong to inspect it, the error condition is enough.

Obtained from: OpenBSD
MFC after: 3 days

Import diff from OpenBSD and remove GNU diff

Some of the modifications from the previous summer of code has been integrated
Modification for compatibility with GNU diff output has been added

Main difference with OpenBSD:
Implement multiple GNU diff options:
* --ignore-file-name-case
* --no-ignore-file-name-case
* --normal
* --tabsize
* --strip-trailing-cr
Make diff -p compatible with GNU diff
Implement diff -l
Make diff -r compatible with GNU diff

Capsicumize diffing 2 regular files
Add a simple test suite

Approved by: AsiaBSDcon devsummit
Obtained from: OpenBSD, GSoC
Relnotes: yes