Implemented a tar-like "--one-file-system" option for metastore

[xkrug-bubeck]

Added a --one-file-system option to stay within the current file system when storing/comparing/applying metadata.
Checks whether the dev_id of the file/dir is the same as the root folder, analog to the same flag in tar.

--one-file-system    Stay within the root folder's file system

[xkrug-bubeck]

This might be interesting when you want to store the metadata of filesystems where you might have mounted devices in subfolders.
Gets rid of any mounts like procfs, sysfs, tmpfs and the automounted USB stick you always forget to unmount.

[przemoc]

Thanks, Jürgen, for another contribution. The idea seems not bad, but I have to look into your patch later to find whether it's done in a way I can accept immediately or we'll have to work on it a bit more.

I guess you can expect various nitpicks. I (almost) always want clean history when taking external contributions, so I may ask you to amend changes to avoid hordes of Fix ... commits. I also don't like when things unrelated to one logical change mentioned in the commit are mixed, so I may also ask you too to split things. I'll reply again after doing basic review.