Feature request: Specify filesystem root to evaluate against

[charles-dyfis-net]

Consider the case where someone has a root filesystem image mounted at /mnt.

Right now, the symlinks tool cannot be usefully used against such an image:

• An absolute link to /foo/bar should only be considered dangling if /mnt/foo/bar does not exist.
• A relative link that tries to reverse upwards past /mnt should be considered to have effect, leaving the referrant at that point in traversal as /mnt.