df: only suppress remote mounts of separate exports with --total

df: only suppress remote mounts of separate exports with --total

* src/df.c (filter_mount_list): Separate remote locations are
generally explicitly mounted, so list each even if they share
the same remote device and thus storage.  However with --total
keep the suppression to give a more accurate value for the
total storage available.
(usage): Expand on the new implications of --total and move
it in the options list according to alphabetic order.
doc/coreutils.texi (df invocation): Mention that --total impacts
on deduplication of remote file systems and also move location
according to alphabetic order.
* tests/df/skip-duplicates.sh: Add remote test cases.
* NEWS: Mention the change in behavior.

Reported in http://bugs.debian.org/737399
Reported in http://bugzilla.redhat.com/920806
Reported in http://bugzilla.opensuse.org/866010
Reported in http://bugzilla.opensuse.org/901905