LU-1203 mdt: recognize old rootsquash and nosquash_nid params

LU-1203 mdt: recognize old rootsquash and nosquash_nid params

Change mdt_process_config() to make it capable of recognizing
old "mdt.rootsquash" and "mdt.nosquash_nid" parameters.

The new parameters are "mdt.root_squash" and "mdt.nosquash_nids".

Signed-off-by: Yu Jian <yujian@whamcloud.com>
Change-Id: I258609ea6f63f14f2e16ec141de1455dd8137b9c
Reviewed-on: http://review.whamcloud.com/3237
Tested-by: Hudson
Reviewed-by: Liang Zhen <liang@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>