MINOR: trace/cli: register the "trace" CLI keyword to list the sources

MINOR: trace/cli: register the "trace" CLI keyword to list the sources

For now it lists the sources if one is not provided, and checks
for the source's existence. It lists the events if not provided,
checks for their existence if provided, and adjusts reported
events/start/stop/pause events, and performs state transitions.
It lists sinks and adjusts them as well. Filters, lock, and
level are not implemented yet.