[mkdir] Implement `-m`

[duckinator]

I've implemented mkdir [-p] DIR, but the -m MODE flag still hasn't been implemented.

mkdir [-p] -m MODE DIR is equivalent to mkdir [-p] DIR && chmod MODE DIR.

I'm unsure if -p + -m results in all created directories having that mode. That should be investigated.

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/mkdir.html