#323 Add option to limit target to filesystem

[Kalle Richter]

It should be possible to limit source directories to filesystem boundaries, e.g. backup / to /mnt/external_disk if another filesystem is mounted at /mnt/external_disk.

experienced with 7.5 on Ubuntu 16.04

[Kalle Richter]

The fact that one can work around this issue by specifying all directories which don't conflict exhaustively is not an option, of course.