Revert r327005 - SPDX tags for license similar to BSD-2-Clause.
After consultation with SPDX experts and their matching guidelines[1],
the licensing doesn't exactly match the BSD-2-Clause. It yet remains to be
determined if they are equivalent or if there is a recognized license that
matches but it is safer to just revert the tags.
Let this also be a reminder that on FreeBSD, SPDX tags are only advisory
and have no legal value (but IANAL).
Pointyhat to: pfg
Thanks to: Rodney Grimes, Gary O'Neall