LU-11461 scripts: Support symlink target

LU-11461 scripts: Support symlink target

Support if configured target is symlink to real device, for instance
/dev/disk/by-id/scsi-WWID.  Also check against bare target for
ZPOOL/DEVICE which will return an empty string when passed to
realpath.
Also fix usage function, so it prints usage and doesn't just error
out.

Test-Parameters: trivial
Signed-off-by: Nathaniel Clark <nclark@whamcloud.com>
Change-Id: I699b1fd36c1e53e99a8d0e6b691374eca42fccc9
Reviewed-on: https://review.whamcloud.com/33277
Reviewed-by: Joe Grund <jgrund@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>