Fix extdom plugin to provide unqualified name in response as sssd expects
    
    extdom plugin handles external operation over which SSSD asks IPA server about
    trusted domain users not found through normal paths but detected to belong
    to the trusted domains associated with IPA realm.
    
    SSSD expects that user or group name in the response will be unqualified
    because domain name for the user or group is also included in the response.
    Strip domain name from the name if getgrnam_r/getpwnam_r calls returned fully
    qualified name which includes the domain name we are asked to handle.
    
    The code already expects that fully-qualified names are following user@domain
    convention so we are simply tracking whether '@' symbol is present and is followed
    by the domain name.