Remove $FreeBSD$: one-line sh pattern

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

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 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