Many fixes for MString, ZString.

testing/mstring_test.cpp

@@ -17,8 +17,9 @@ int main(int argc, char **argv) {
    test3 = 'a';
    std::cout << "char operator=: [" << test3 << "]." << std::endl;   
 
+   coreutils::ZString test7("zstring data");
    coreutils::MString test4;
-   test4 << "this is a test.";
+   test4 << "this is a test." << test7;
    std::cout << "char* operator<<: [" << test4 << "]." << std::endl;   
    test4 << "another test " << 75;
    std::cout << "char* operator<<: [" << test4 << "]." << std::endl;