PAC: if user entry already exists keep it
    
    Currently the PAC responder deletes a user entry and recreates it if
    some attributes seems to be different.
    
    Two of the attributes where the home directory and the shell of the
    user. Those two attributes are not available from the PAC but where
    generates by the PAC responder. The corresponding ID provider might have
    better means to determine those attributes, e.g. read them from LDAP, so
    we shouldn't change them here.
    
    The third attribute is the user name. Since the PAC responder does
    lookups only based on the UID we can wait until the ID provider updates
    the entry.
    
    Fixes https://fedorahosted.org/sssd/ticket/1996