filesystem code should have an option not to filter duplicate, and/or log dropped duplicates

[fearful-symmetry]

Right now, GetFilesystems will run all filesystems through a filterDuplicates function that uses the device name to remove duplicate entries. For most instances this makes sense, but for certain device mapper setups, where a user might have multiple directories backed by a single device, this can be confusing. We might want some kind of filter_duplicates flag to enable/disable this behavior, or at least, a debug-level log line that tells the user when a filesystem has been filtered out.