Handle devices that are mounted at multiple paths

[sgoll]

This PR improves processing when findmnt returns more than a single mount path for a device. This may happen when the same device has been mounted multiple times, e.g. via bind mounts or btrfs subvolume mounts.

The menu shows each mount path in brackets. In the submenu there is a separate line for each mount path. When all devices have only a single mount path, the output is the same as before.

Fixes #31