style: Recomend SPDX licenses

While here, take this opportunity to update the copyright and clean
some mdoc errors.

MFC after: 3 days
Reviewed by: carlavilla, mhorne, imp
Approved by: imp (srcmgr)
Approved by: carlavilla, mhorne (mentors)
Differential Revision: https://reviews.freebsd.org/D49051