Implement our own version of wcsrtombs().

* add WcharStringToMultibyte() to libroot's locale backend
* implement wcstombs(), wcsrtombs() and wcsnrtombs() on top of that
  new backend function

headers/private/libroot/locale/ICUCtypeData.h

@@ -39,6 +39,11 @@ public:
 									size_t& lengthOut);
 			status_t			WcharToMultibyte(char* mbOut, wchar_t wc,
 									mbstate_t* mbState, size_t& lengthOut);
+			status_t			WcharStringToMultibyte(char* mbDest,
+									size_t mbDestLength,
+									const wchar_t** wcSource,
+									size_t wcSourceLength, mbstate_t* mbState,
+									size_t& lengthOut);
 
 			const char*			GetLanginfo(int index);
 

headers/private/libroot/locale/ICULocaleBackend.h

@@ -50,6 +50,11 @@ public:
 									size_t& lengthOut);
 	virtual status_t			WcharToMultibyte(char* mbOut, wchar_t wc,
 									mbstate_t* mbState, size_t& lengthOut);
+	virtual status_t			WcharStringToMultibyte(char* mbDest,
+									size_t mbDestLength,
+									const wchar_t** wcSource,
+									size_t wcSourceLength, mbstate_t* mbState,
+									size_t& lengthOut);
 
 	virtual	const char*			GetLanginfo(int index);
 

headers/private/libroot/locale/LocaleBackend.h

@@ -132,6 +132,11 @@ public:
 									size_t& lengthOut) = 0;
 	virtual status_t			WcharToMultibyte(char* mbOut, wchar_t wc,
 									mbstate_t* mbState, size_t& lengthOut) = 0;
+	virtual status_t			WcharStringToMultibyte(char* mbDest,
+									size_t mbDestLength,
+									const wchar_t** wcSource,
+									size_t wcSourceLength, mbstate_t* mbState,
+									size_t& lengthOut) = 0;
 
 	virtual	const char*			GetLanginfo(int index) = 0;