games/open-adventure: upgrade to version 1.21
Changes relative to the previous port version 1.16:
- Saying Z’ZZZ at reservoir no longer causes the waters to part and crash.
- Man-page has been brought up-to-date.
- Ensure that the KNIVES_VANISH message can’t issue twice.
- Make oldstyle correctly suppress line editing.
- Code-hardening and spell-checking of texts.