df: work around long-named /dev/disk/by-uuid/... symlinks

On systems with recent kernel/tools, a symlink from /etc/mtab to
/proc/mounts, and a by-UUID mount (i.e., soon, nearly everyone),
you will see something like the following when running "df -hT":
(this has been truncated to fit in a width-limited ChangeLog file)

Filesystem                                             Type      Siz...
rootfs                                                 rootfs     11G
udev                                                   devtmpfs  3.8G
tmpfs                                                  tmpfs     774M
/dev/disk/by-uuid/828fc648-9f30-43d8-a0b1-f7096a2edb66 ext4       11G
tmpfs                                                  tmpfs     1.6G
/dev/sda2                                              ext3      494M
/dev/sda5                                              ext4       12G
/dev/sda6                                              ext4      9.9G

Contrast that with what we're used to seeing (modulo the
two entries mounted on "/", which is a separate problem):

Filesystem     Type      Size  Used Avail Use% Mounted on
rootfs         rootfs     11G  1.9G  8.0G  19% /
udev           devtmpfs  3.8G     0  3.8G   0% /dev
tmpfs          tmpfs     774M  376K  774M   1% /run
/dev/sda3      ext4       11G  1.9G  8.0G  19% /
tmpfs          tmpfs     1.6G  8.0K  1.6G   1% /run/shm
/dev/sda2      ext3      494M   78M  392M  17% /boot
/dev/sda5      ext4       12G  7.6G  3.7G  68% /usr
/dev/sda6      ext4      9.9G  6.6G  2.8G  71% /var

When that long /dev/disk/by-uuid/... name is merely a symlink
to a much shorter (and often more useful) device name like
"/dev/sda3", and when it's part of a listing of all file systems,
I would much prefer to see only the latter.  Similarly, when using
an encrypted root file system, you would see a name like
/dev/mapper/luks-828fc648-9f30-43d8-a0b1-f7196a2edb66 pointing
to say, /dev/dm-0, I prefer the shorter name.

I.e., if I explicitly run
"df -hT /dev/disk/by-uuid/828fc648-9f30-43d8-a0b1-f7096a2edb66",
then, it's fine -- and expected -- to print to the long name.
It was explicitly given.  However, with no non-option argument,
df should print the shorter name.  Note that performing this
translation at a lower level (via a change to gnulib's mountlist.c)
would make it impossible to distinguish those two cases.

* src/df.c: Include "canonicalize.h".
(get_dev): Add a parameter, telling when we're in process-all-
mount-points mode; update all callers.  When true, resolve
UUID-suffixed symlinks.
* NEWS (Changes in behavior): Mention it.
Reported by Dan Jacobson in http://bugs.gnu.org/10363

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