MFC S340428: Prepare move of ctm from base to a port (misc/ctm) by:

• Adding a note to UPDATING
• Adding a note to the history section of the manpage ctm.1
• Adding a message printed to STDERR to the ctm program

This version is meant for release in FreeBSD-12.0 and should remain in
FreeBSD-12 over its life-time.

A follow-up commit will remove ctm from -CURRENT after the MFC to 12
has happened.

Approved by: re
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D17969