diff3: Move to own package

diff3 is the last remaining GPL-licensed userland component. Move it
to its own package, so that it may be excluded if desired.

Reviewed by: ivy
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52810

(cherry picked from commit e7258c42520c18c034f644b97377d8d2a0ad8b7c)