LU-5170 utils: add lfs df -H for decimal units

LU-5170 utils: add lfs df -H for decimal units

Running "lfs df -ih" prints a base-two suffix for inode counts,
which is somewhat unintuitive (e.g. 100000 becomes 97.2K inodes).
While this is consistent with upstream "df", it also has a "-H"
option to print the output with decimal suffixes.

Add the -H/--si option to "lfs df" also.

Document the 'f' (flash) and 'N' (noprecreate) flags for "lfs df".

Lustre-change: https://review.whamcloud.com/41271
Lustre-commit: 7b720df1fbd4136cd1ab8f3fefefd3971b2f7031

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I06b8df4ae2940107720e57013bf187b3473ebbe5
Reviewed-by: Olaf Faaland-LLNL <faaland1@llnl.gov>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/48267
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>