manager: if called with compat telinit interface, tell users to update

manager: if called with compat telinit interface, tell users to update

In https://bugzilla.redhat.com/show_bug.cgi?id=2479961 a user
reported that they are confused that 'init 6' and such commands
do not work anymore. We removed support for the whole interface,
but it's likely that such commands persist in various scripts
and finger memories. Let's give a helpful hint that this inteface
is gone and what to use instead.