Porter's handbook: GPL licenses installation

As 13.20 kindly asks to avoid installing more GNU General Public License
files into our system,

• Repeat this recommendation in 5.7.
• Modify an example in 5.7 going against that recommendation by replacing GPLv3+ with BSD2CLAUSE.

Reviewed by: Pau Amma <pauamma@gundo.com>
Approved by: gerald (mentor), doc (carlavilla)
Differential Revision: https://reviews.freebsd.org/D35127