license: Move to BSD-2-Clause and drop -FreeBSD

BSD-2-Clause-FreeBSD has been deprecated as a separate
license. https://tools.spdx.org/app/check_license/ says that the text
matches BSD-2-Cluase. There's some history behind the mismatch, but it's
best to move forward now that our preferred license matches
BSD-2-Clause.

Sponsored by: Netflix

Reviewed by: pauamma, andrew
Differential Revision: https://reviews.freebsd.org/D37714