Implement debuglog command

Add debuglog command that takes toggle|on|off. Add get_debug_logging()
to be able to toggle. Make t/187-commands-parser.t expect 'debuglog'.
Document the debuglog command in userguide.

include/log.h

@@ -50,6 +50,12 @@ void open_logbuffer(void);
  */
 void close_logbuffer(void);
 
+/**
+ * Checks if debug logging is active.
+ *
+ */
+bool get_debug_logging(void);
+
 /**
  * Set debug logging.
  *