LUDOC-377 misc: cleanup of root squash settings

LUDOC-377 misc: cleanup of root squash settings

Specify how to retrieve root squash settings using
lctl get_param and also document the set_param option
in addition to the conf_param settings already described.

Adds a note to mention nodemaps as an alternative.

Signed-off-by: Joseph Gmitter <joseph.gmitter@intel.com>
Change-Id: I753296f097b0006b084e8e0e7e11c369df3105ee
Reviewed-on: https://review.whamcloud.com/27954
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>