Enhance PAC responder for AD users
    
    This patch modifies the PAC responder so that it can be used with the AD
    provider as well. The main difference is that the POSIX UIDs and GIDs
    are now lookup up with the help of the SID instead of being calculated
    algorithmically. This was necessary because the AD provider allows
    either algorithmic mapping or reading the value from attributes stored
    in AD.
    
    Fixes https://fedorahosted.org/sssd/ticket/1558