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

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