cu.1: describe better + tag spdx

Preserve over 40 years of "call UNIX" BSD heritage
while answering "one line about what it does" e.g.

"how do I get a serial console?"
"% apropos serial"

MFC after: 3 days
Reported by: imp

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1423