If DB contains only one path, default to that path if one is not specified when using ls, ui, or the like

[felder]

While not super essential, a nice feature would be for duc to assume a default for the path if the db only contains a single top level path and no path was specified.

This prevents having to separately query the database to find a path to specify.

For example:

# duc info -d filestore.db
Date       Time       Files    Dirs    Size Path
2023-08-18 21:04:14       0      13   52.0K /export/filestore
# duc ls -d filestore.db
Path /export/reports not found in database
Requested path not found
Segmentation fault (core dumped)
# duc ls -d filestore.db /export/filestore
 48.0K filestore

It just seems less useful for it to assume the cwd is the path when that is not in the index.

Also I assume the segfault is a result of #236