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

linux/Platform.c

@@ -376,7 +376,7 @@ void Platform_getNetworkIO(unsigned long int *bytesReceived,
                      &packetsTransmittedParsed) != 5)
          continue;
 
-      if (0 == strcmp(interfaceName, "lo:"))
+      if (String_eq(interfaceName, "lo:"))
          continue;
 
       bytesReceivedSum += bytesReceivedParsed;