bectl: Use SPDX only license
bectl.8 had a truncated license without the required disclaimer.
Correct it consistently by converting all bectl licenses to SPDX form.
(not yet) Approved by: kevans
(not yet) Approved by: Kyle Kneitinger <kyle@kneit.in>
(not yet) Approved by: Wes Maag <wes@jwmaag.org>