Allow os.FileInfo impls to provide a *file.FileInfo

[ozkatz]

Use case: I have custom logic to determine the NFS File ID. Currently, the only to return it to clients is by returning a syscall.Stat_t from Sys(), which cannot work on Windows machines.

This PR introduces a small interface, which, if implemented by the os.FileInfo implementation, allows returning an explicit *file.FileInfo. If unimplemented, existing logic remains, so should be backwards compatible.

[willscott]

I wonder if it would be more direct to always call Sys() and then see if that is direct our FileInfo struct before attempting the platform-specific cast?

[ozkatz]

Done

[willscott]

Thanks!