LU-3527 nodemap: idmap management functions

LU-3527 nodemap: idmap management functions

This adds support for uid/gid mapping management to the nodemap
module. This mapping is added as four rbtrees in the nodemap
structure for forward and reverse mapping of UIDs and GIDs.
Postorder traversal for rbtrees have been added for cleanup
upon nodemap removal.

lctl commands have been added for adding, removing, and testing
maps.

Unit tests have been added to test the maps in all the relevant
modes (allow root access, trust client ids).

Signed-off-by: Joshua Walgenbach <jjw@iu.edu>
Change-Id: I9a8672f2883185a35d3553e5c2e91ae1d2cb094d
Reviewed-on: http://review.whamcloud.com/8125
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Andrew Perepechko <andrew_perepechko@xyratex.com>
Reviewed-by: Ken Hornstein <kenh@cmf.nrl.navy.mil>