default channels should not be created when a channel has been explicitely created
As noted in bug https://bugs.lttng.org/issues/516 , creating a "default" channel if a user forgets to put "-c channel_name" in the following add-context, enable-event, etc commands is misleading. The natural expected behavior would be that the command would apply the the last enabled channel (notion of a "current" channel, like we have for sessions), but certainly not that it creates a new channel with default attributes behind the user's back.
An intermediate fix that would not require to keep a notion of current channel could be to refuse creation of a default channel if at least one non-default channel has already been created in the session.
Since this is an issue of UI correctness (it's hiding a side-effect from the user), I think it should be treated as a bug.