Provide access to data under /proc/[pid]/fdinfo/

In order to implement a file-progress tool I would like access to the information under `/proc/[pid]/fdinfo/` - specifically the file descriptor's position `pos`. According to the [support.md](https://github.com/eminence/procfs/blob/master/support.md) file this is not yet implemented and indeed I find no such features in the docs or code.

I'd be happy to raise a PR for this, but could use a steer on naming; the `procfs::process::FDInfo` struct was what I expected to represent this data, but it seems to be constructed directly from the file symlinks under `/proc/[pid]/fd/` instead.

Suggestions for naming and/or am I missing anything?