Bug#807456: mention how to abort, right there in the message

[積丹尼 Dan Jacobson]

And alas
# systemctl  set-property dictd.service TimeoutStopUSec=89sec
Unknown assignment TimeoutStopUSec=89sec.
one cannot change them except by editing each of e.g., /etc/init.d/dictd
individually.

Also Unknown assignment should be Readonly value.