LU-7506 lfs: "lfs quota -h" should support petabytes output

LU-7506 lfs: "lfs quota -h" should support petabytes output

This patch adds human-readable petabytes output and allows to set
fraction block limit, e.g. 1.5P.
Also, it adds sanity-quota.sh test_27d to verify these new functions,
and replaces some sprintf() with snprintf() to meet the code style
requirement.

Signed-off-by: Emoly Liu <emoly.liu@intel.com>
Change-Id: I1dced352687e8e53988f85d2781396e647b62865
Reviewed-on: http://review.whamcloud.com/17441
Tested-by: Jenkins
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>