* refactor: rename enum types in ip-cache private enum class is_updating_t
this avoids name collisions with YES and NO on macOS
* refactor: rename tr_strv_convert_utf8() to tr_strv_to_utf8_string()
use a name that is symmetrical with the upcoming tr_strv_to_utf8_nsstring()
* feat: add tr_strv_to_utf8_nsstring()
* test: add tests for the new tr_strv_to_utf8_nsstring() methods