XUtils string related updates

- allow count out-parameter of String_split() to be NULL
- introduce xStrndup()
- do not allow NULL pointers passed to String_eq()
  it is not used in any code
- implement String_startsWith(), String_contains_i() and String_eq()
  as inline header functions
- adjust several conversion issues

CRT.c

@@ -655,7 +655,7 @@ void CRT_init(int delay, int colorScheme, bool allowUnicode) {
    setlocale(LC_CTYPE, "");
 
 #ifdef HAVE_LIBNCURSESW
-   if (allowUnicode && strcmp(nl_langinfo(CODESET), "UTF-8") == 0)
+   if (allowUnicode && String_eq(nl_langinfo(CODESET), "UTF-8"))
       CRT_utf8 = true;
    else
       CRT_utf8 = false;