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

Action.h

@@ -7,6 +7,8 @@ Released under the GNU GPLv2, see the COPYING file
 in the source distribution for its full text.
 */
 
+#include "config.h" // IWYU pragma: keep
+
 #include <stdbool.h>
 #include <sys/types.h>