aufs: mvdown, aux error code for userspace

AUFS_CTL_MVDOWN ioctl can return an auxiliary error code for userspace.
The actual message string is defined in aufs-util.git.

Signed-off-by: J. R. Okajima <hooanon05@yahoo.co.jp>
aufs: mvdown, userspace can be verbose

By previous commit,
d84622f 2013-08-19 aufs: mvdown, aux error code for userspace
aumvdown utility in aufs-util.git can be verbose now.
So the kernel-space can be less verbose, but return some additional info
(branch indexes for source and destination of move-down).

Signed-off-by: J. R. Okajima <hooanon05@yahoo.co.jp>
aufs: tiny, mvdown, reduce the stack consumption

Signed-off-by: J. R. Okajima <hooanon05@yahoo.co.jp>
aufs: tiny, wrap around a long line

Signed-off-by: J. R. Okajima <hooanon05@yahoo.co.jp>
aufs: for mvdown, a new flag for cpup, OVERWRITE

In order to implement the "overwrite lower" operation, introduce a new
flag OVERWRITE for copy-up/down routines.

Signed-off-by: J. R. Okajima <hooanon05@yahoo.co.jp>
aufs: mvdown, the "overwriting lower" option

By this option, move-down operation overwrites the existing target file
on lower branch.

Signed-off-by: J. R. Okajima <hooanon05@yahoo.co.jp>
aufs: mvdown, the "keep upper" option

By this option, the move-down operation becomes copy-down actually.

Signed-off-by: J. R. Okajima <hooanon05@yahoo.co.jp>
Merge remote-tracking branch 'aufs3-standalone/aufs3.9/00base' into aufs3.9/10static
Merge branch 'aufs3.9/10static' into aufs3.9/11proc_map
Merge branch 'aufs3.9/11proc_map' into aufs3.9/18public