AddNamespaceRangesUser takes a username and uses the standard system utility to create a system user/group pair used to hold the /etc/sub{uid,gid} ranges which will be used for user namespace mapping ranges in containers.

LoadIdentityMapping takes a requested username and using the data from /etc/sub{uid,gid} ranges, creates the proper uid and gid remapping ranges for that user/group pair.

LookupGID uses traditional local system files lookup (from libcontainer/user) on a group ID, followed by a call to `getent` for supporting host configured non-files passwd and group dbs.

LookupGroup uses traditional local system files lookup (from libcontainer/user) on a group name, followed by a call to `getent` for supporting host configured non-files passwd and group dbs.

LookupUID uses traditional local system files lookup (from libcontainer/user) on a uid, followed by a call to `getent` for supporting host configured non-files passwd and group dbs.

LookupUser uses traditional local system files lookup (from libcontainer/user) on a username, followed by a call to `getent` for supporting host configured non-files passwd and group dbs.