Textual information from /proc should not be exposed as String

The kernel does not care about proper UTF-8 encodings, so conversion into `String` are always lossy operations.
`Process.cmdline()` should return a `Vec`, `Stat.comm` should be an `OsString`. These are the only examples I noticed when checking whether I could replace my own code with `procfs` – there are probably quite a few other examples.

Would you accept a PR that replaces those uses of `String` with `OsString`? I am aware that this would require an API version bump – if you don't want to do that, I could also do a patch that only adds fields and methods.