Add the "btrfs filesystem label" command

Add the "btrfs filesystem label" command

Hi all,

this patch adds the command "btrfs filesystem label" to change (or show) the
label of a filesystem.
This patch is a subset of the one written previously by Morey Roof. I
included the user space part only. So it is possible only to change/show a
label of a *single device* and *unounted* filesystem.

The reason of excluding the kernel space part, is to simplify the patch in
order to speed the check and then the merging of the patch itself. In fact I
have to point out that in the past there was almost three attempts to propose
this patch, without success neither complaints.

Chris, let me know how you want to proceed. I know that you are very busy,
and you prefer to work to stabilize btrfs instead adding new feature. But I
think that changing a label is a *essential* feature for a filesystem
managing tool. Think about a mount by LABEL.

To show a label

$ btrfs filesystem label <device>

To set a label

$ btrfs filesystem label <device> <newlabel>

Please guys, give a look to the source.
Comments are welcome.

You can pull the source from the branch "label" of the repository
http://cassiopea.homelinux.net/git/btrfs-progs-unstable.git

Regards
G.Baroncelli

Signed-off-by: Chris Mason <chris.mason@oracle.com>