std::fs: change ownership or Improvements for UNIX Domain Socket creation.

[cheako]

https://gitlab.com/cheako/files-rs/blob/c993fc0ce90fcacc1b8821035027baf623366ab8/src/main.rs#L72-127

If there was an interface to assist with changing ownership that would make this code nicer. I can only speak about UNIX systems, not including Mac and even then only about using libc.