Attached.

Background/context:

* http://www.openwall.com/lists/musl/2018/01/18/4
* http://austingroupbugs.net/view.php?id=169#c283

Seems to work on basic usage, but has not yet been thoroughly tested/vetted.
Sharing in case useful / good starting point regardless ;).

Also available on github:
https://github.com/dtzWill/musl/tree/feature/strptime-s-fmt

~Will