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.

Approved by: kevans
Approved by: Kyle Kneitinger <kyle@kneit.in>
Approved by: Wes Maag <wes@jwmaag.org>
Approved by: mhorne (mentor, implicit)
Differential Revision: https://reviews.freebsd.org/D49066