df: also deduplicate virtual file systems

* src/df.c (filter_mountlist): Remove the constraint that
a '/' needs to be in the device name for a mount entry to
be considered for deduplication.  Virtual file systems also
have storage associated with them (like tmpfs for example),
and thus need to be deduplicated since they will be shown
in the default df output and subject to --total processing also.
* test/df/skip-duplicates.sh: Add a test to ensure we deduplicate
all entries, even for virtual file systems.  Also avoid possible
length operations on many remote file systems in the initial
check of df operation.  Also avoid the assumption that "/root"
is on the same file system as "/".
* NEWS: Mention the change in behavior.

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