abell-94 license

[Frédéric Fauberteau <triaxx%NetBSD.org@localhost>]

Hi folks,

I just commit the abell-94 that is used by sysutils/lsof and that is a 
derived from spencer-94 (https://spdx.org/licenses/Spencer-94.html).

I have (too many quickly) added it to DEFAULT_ACCEPTABLE_LICENSES but as 
noticed by Leonardo, this license is not directly Open Source/FSF 
compliant.

If someone has any objection about that, I can revert my commit on 
mk/license.mk