s4-auth Add function to obtain any user's session_info from a given LDB

s4-auth Add function to obtain any user's session_info from a given LDB

This will be a building block for a tokenGroups test, which can
compare against a remote server (in particular the rootDSE) against
what we would calculate the tokenGroups to be.

(this meant moving some parts out of the auth_sam code into the
containing library)

Andrew Bartlett