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 sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

diff: link with libm for sqrt()

Reported by: Jenkins
Fixes: bcf2e78dc48378456798191f1c15cb76d6221a65

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

Add HAS_TESTS to all Makefiles that are currently using the
`SUBDIR.${MK_TESTS}+= tests` idiom.

This is a follow up to r321912.

Convert traditional ${MK_TESTS} conditional idiom for including test
directories to SUBDIR.${MK_TESTS} idiom

This is being done to pave the way for future work (and homogenity) in
^/projects/make-check-sandbox .

No functional change intended.

MFC after: 1 weeks

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

Check in first src/tests snapshot from NetBSD anoncvs

Sources were obtained like so:

% export CVSROOT="anoncvs@anoncvs.NetBSD.org:/cvsroot"
% cvs -z9 co -D "09/30/2014 20:45" -P src/tests
% mv src/tests/* tests/dist/.

'*CVS*' has been added to svn:ignore to ease updating periodically from
upstream

Some line ending issues had to be resolved with test outputs and scripts
via dos2unix and by deleting the eol-style property set in usr.bin/sort

Discussed with: rpaulo
Sponsored by: EMC / Isilon Storage Division