Fixes to our LDAP/vampire codepaths:

Fixes to our LDAP/vampire codepaths:

 - Try better to add the appropriate mapping between UID and SIDs, based
   on Get_Pwnam()
 - Look for previous users (lookup by SID) and correctly modify the existing
   entry in that case
 - Map the root user to the Admin SID as a 'well known user'
 - Save the LDAPMessage result on the SAM_ACCOUNT for use in the next 'update'
   call on that user.  This means that VL's very nice work on atomic LDAP
   updates now really gets used properly!
 - This also means that we know the right DN to update, without the extra
   round-trips to the server.

Andrew Bartlett
(This used to be commit c7118cb31dac24db3b762fe68ce655b17ea102e0)