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.

src/messages.h

@@ -26,6 +26,8 @@
 /** Cannot create writer thread */
 #define LIBISO_THREAD_ERROR         0x00030110
 
+/** Charset conversion error */
+#define LIBISO_CHARSET_ERROR        0x00030500
 
 /** Can't read file (ignored) */
 #define LIBISO_CANT_READ_FILE		0x00031001