LU-1581 utils: label methods

LU-1581 utils: label methods

label methods for ldiskfs and zfs, used by mount utility
to update the label upon the first successful mount.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: Ibd5e14bd10e39d71264d15bbee42021b089091ea
Reviewed-on: http://review.whamcloud.com/3611
Tested-by: Hudson
Reviewed-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Oleg Drokin <green@whamcloud.com>