Replacing GNU diff

[nia <nia%NetBSD.org@localhost>]

Hi all, I've been working on a patch to import diff and diff3 from
FreeBSD to replace GNU diffutils in base (when MKBSDDIFF is enabled).

I chose FreeBSD diff as a basis rather than OpenBSD diff primarily
because more work has been done to support GNU extensions, making
it essentially feature-compatible with our GPLv2 diff. OpenBSD diff3
unfortunately also relies on ksh.

Work was done to remove or port FreeBSD-specific code, avoid the need
for defining _OPENBSD_SOURCE to compile it, fixing some typing problems
so that it compiles with WARNS. A couple of features were also removed
since they are beyond the scope for replacing GPLv2 diff and complicate
the code.

The patch that integrates the result into our build system is available
here:

https://cdn.NetBSD.org/pub/NetBSD/misc/nia/diffimport.20251128.patch.gz

(also available over plain ftp for your vax)

It seems to work well. A future task is porting some of the ATF tests
from FreeBSD also.