Serious work on JString and some minor MString ajustments.

MString.h

@@ -227,10 +227,12 @@ namespace coreutils {
       MString &read(int fd);
       
       int offset();
+
+    protected:
+      void setSize(int size);
       
     private:
       int bufferSize = 0;
-      void setSize(int size);
    };
    
 }