Add FileStat()

[takebayashi]

Add FileStat(), that may be useful for implementing external log-rotation managers.

[Nino-K]

@takebayashi Thanks for your contribution, could you elaborate a bit on what is this trying to solve or perhaps how should it be used?

Thanks,
Nino

[takebayashi]

@Nino-K I'm trying to implement the persistence of the status/position of tailing to resume tailing after restart. File stats info (such as inode value) are needed to detect log-rotations while the tailing process is not running.