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

[dcminter]

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 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?