df: prefer fullpath entries when deduplicating

* src/df.c (struct devlist): Add a new element for storing
pointers to mount_entry structures.
(devlist_head, dev_examined): Remove.
(filter_mount_list): Add new function to filter out the rootfs
entry (unless -trootfs is specified), and duplicities. The
function favors entries with a '/' character in me_devname
or those with the shortest me_mountdir string, if multiple
entries fulfill the first condition.
Use struct devlist to build up a list of entries already known,
and finally rebuild the global mount_list.
(get_all_entries): Call the above new function unless the -a
option is specified.
(get_dev): Remove the code for skipping rootfs and duplicities.
* tests/df/skip-duplicates.sh: Add test cases.

Co-authored-by: Bernhard Voelker <mail@bernhard-voelker.de>

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