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Aufs pseudo-link (plink) represents a virtual hardlink across the
branches. To implement the plink maintenance mode, aufs uses procfs.
See also the document in this commit.

There is an external user-space utility called 'auplink' in
aufs-util.git, which has these features.
- 'list' shows the pseudo-linked inode numbers and filenames.
- 'cpup' copies-up all pseudo-link to the writable branch.
- 'flush' calls 'cpup', and then 'mount -o remount,clean_plink=inum'
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>