Add trim to ZString.

testing/zstring_test.cpp

@@ -91,5 +91,8 @@ int main(int argc, char **argv) {
    std::cout << test16.substring(10, 4) << std::endl;
    std::cout << test16.substring(0, 7) << std::endl;
    std::cout << test16.substring(18, 20) << std::endl;
+
+   coreutils::ZString test17("   hello there   ");
+   std::cout << "[" << test17.trim() << "]" << std::endl;
    
 }