Add --dereference option to df

[Ondrej Oprala]

Hi, this patch adds the --dereference option to df, so if a symlink is specified
as an argument to df (possibly /dev/disk/by-uuid/*), df outputs info about the
filesystem the symlinked file is on instead. Now, I realize the option's name is
a bit misleading, since the arguments are stat-ed anyway, but IMHO that's 
exactly
what it looks like from a user's perspective.

Thanks,
Ondrej

df-dereference.patch
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