Let users(1) use an std::set, instead of std::{vector,sort,unique}.

Reviewed by:	gahr
Don't use auto, as we also need to support GCC 4.2.
Fix a couple of style nits.

- Use set instead of std::set, to be consistent with the rest of the file.
- Remove return (0); it's not required.
- Add a dash at the beginning of the copyright, per style(9).