Carefully process desired name based on service
    
    In case the name type is GSS_C_NT_STRING_UID_NAME or GSS_NT_MACHINE_UID_NAME
    we want to be able to impersonate the user referenced by the uid.
    
    This is allowed exclusively for trusted services otherwise a generic
    unprivileged application would be allowed to impersonate any user if there are
    credentials available on the system or client keytabs installed.
    
    Signed-off-by: Simo Sorce <simo@redhat.com>
    Reviewed-by: Günther Deschner <gdeschner@redhat.com>