Support for setting an output charset for RR NM entries.

It will default to input charset (i.e. the locale charset for now). 
Names will be stored internally in that locale charset. Note that input 
charset musn't be changed by user. Instead, we can provide an input 
charset property to IsoFilesystem implementations.

demo/iso.c

@@ -39,7 +39,8 @@ int main(int argc, char **argv)
         0, /* dir_mode */
         0, /* file_mode */
         0, /* uid */
-        0  /* gid */
+        0, /* gid */
+        NULL /* output charset */
     };
 	
     if (argc < 2) {