Add implementation of mbsrtowcs() to our locale backend.

* add MultibyteStringToWchar() to ICU locale backend
* implement mbsrtowcs() and mbsnrtowcs() on top of 
  MultibyteStringToWchar()
* drop respective glibc files

headers/private/libroot/locale/ICUCtypeData.h

@@ -33,6 +33,10 @@ public:
 			status_t			MultibyteToWchar(wchar_t* wcOut, const char* mb,
 									size_t mbLength, mbstate_t* mbState,
 									size_t& lengthOut);
+			status_t			MultibyteStringToWchar(wchar_t* wcDest,
+									size_t wcDestLength, const char** mbSource,
+									size_t mbSourceLength, mbstate_t* mbState,
+									size_t& lengthOut);
 			status_t			WcharToMultibyte(char* mbOut, wchar_t wc,
 									mbstate_t* mbState, size_t& lengthOut);
 

headers/private/libroot/locale/ICULocaleBackend.h

@@ -44,6 +44,10 @@ public:
 	virtual status_t			MultibyteToWchar(wchar_t* wcOut, const char* mb,
 									size_t mbLength, mbstate_t* mbState,
 									size_t& lengthOut);
+	virtual status_t			MultibyteStringToWchar(wchar_t* wcDest,
+									size_t wcDestLength, const char** mbSource,
+									size_t mbSourceLength, mbstate_t* mbState,
+									size_t& lengthOut);
 	virtual status_t			WcharToMultibyte(char* mbOut, wchar_t wc,
 									mbstate_t* mbState, size_t& lengthOut);
 

headers/private/libroot/locale/LocaleBackend.h

@@ -126,6 +126,10 @@ public:
 	virtual status_t			MultibyteToWchar(wchar_t* wcOut, const char* mb,
 									size_t mbLength, mbstate_t* mbState,
 									size_t& lengthOut) = 0;
+	virtual status_t			MultibyteStringToWchar(wchar_t* wcDest,
+									size_t wcDestLength, const char** mbSource,
+									size_t mbSourceLength, mbstate_t* mbState,
+									size_t& lengthOut) = 0;
 	virtual status_t			WcharToMultibyte(char* mbOut, wchar_t wc,
 									mbstate_t* mbState, size_t& lengthOut) = 0;