In the earlier revision of the keygrabber PR, there was a `release_key`
and it was suggested to rename it `stop_key`. However its sibling
`release_event` wasn't, so it is now confusing.
The commit adds a mild deprecation codepath to avoid breaking configs
based on git-master. However it isn't a "long term" deprecation notice
and the code can probably be removed in 5.0 without further delay.
Emmanuel Lepage Vallee