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.
diff --git a/include/proto/trace.h b/include/proto/trace.h
index a299237..24d2601 100644
--- a/include/proto/trace.h
+++ b/include/proto/trace.h
@@ -34,6 +34,20 @@
 extern struct list trace_sources;
 extern THREAD_LOCAL struct buffer trace_buf;
 
+/* return a single char to describe a trace state */
+static inline char trace_state_char(enum trace_state st)
+{
+	return (st == TRACE_STATE_RUNNING) ? 'R' :
+	       (st == TRACE_STATE_WAITING) ? 'w' :
+	                                     '.';
+}
+
+/* return a single char to describe an event state */
+static inline char trace_event_char(uint64_t conf, uint64_t ev)
+{
+	return (conf & ev) ? '+' : '-';
+}
+
 /* registers trace source <source>. Modifies the list element!
  * The {start,pause,stop,report} events are not changed so the source may
  * preset them.