Handle log messages from build-remote

This makes the progress indicator show statuses like "connecting to
'root@machine'".

src/libutil/logging.hh

@@ -130,6 +130,11 @@ extern Logger * logger;
 
 Logger * makeDefaultLogger();
 
+Logger * makeJSONLogger(Logger & prevLogger);
+
+bool handleJSONLogMessage(const std::string & msg,
+    const Activity & act, std::map<ActivityId, Activity> & activities);
+
 extern Verbosity verbosity; /* suppress msgs > this */
 
 /* Print a message if the current log level is at least the specified