Added ZString.c_str() function; Added MStrin operator=.

File.cpp

@@ -27,6 +27,10 @@ namespace coreutils {
       }
    }
 
+   File::File(coreutils::ZString &fileName, int mode, int authority) {
+      File(fileName.str(), mode, authority);
+   }   
+
    File::~File() {
       close(fd);
       setBufferSize(0);