df: do not print duplicate entries and rootfs by default

* src/df.c (struct devlist): Add new struct for storing already-
examined device numbers.
(devlist_head): Add new store of the above type.
(show_rootfs): Add new global boolean to not skip rootfs.
(dev_examined): Add new function to check if the device has
already been traversed.
(get_dev): Filter out rootfs unless "-t rootfs" or the -a
option is specified. Filter out duplicate entries by calling
the above new dev_examined unless the -a option is specified.
(main): Set the show_rootfs variable appropriately when the -t
option is specified for rootfs. Free device list (guarded by
IF_LINT).
* tests/df/skip-duplicates.sh: Add test to exercise the skipping
of duplicate entries.
* tests/df/skip-rootfs.sh: Add test to exercise the skipping
of the rootfs pseudo file system.
* tests/local.mk: Add the above new tests.
* NEWS (Changes in behavior): Mention the changes.
* doc/coreutils.texi (df invocation): Document df's behavior about
skipping rootfs and duplicate entries.

Co-authored-by: Bernhard Voelker.

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