EX-3658 pumount: refine exit statuses

EX-3658 pumount: refine exit statuses

   0  Unmount succeeded and no processes using FS remained.
   1  Unmount succeeded but one or more processes using FS remained.
   2  Unmount failed or some other major error occurred.
   3  An invalid option or argument was supplied.

Add sanity-pumount 57a-c to verify.

Test-Parameters: trivial testlist=sanity-pumount clientextra_install_params="--packages pumount"
Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: I7d0d67129af08d5c56ab5e676d4d92b3a14f2d9d
Reviewed-on: https://review.whamcloud.com/44758
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/44958
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>