pid1: ignore whole /run/host hierarchy
    
    Let's mark the whole /run/host hierarchy as something to ignore by PID 1
    for generation of .mount units, i.e. consider it as "extrinsic".
    
    By unifying container mgr supplied resources in one dir it's also easy
    to exclude the whole lot from PID1's management inside the container.
    This is the right thing to do, since from the payload's PoV these mounts
    are just API and not manipulatable as they are established, managed and
    owned by the container manager, not the payload.
    
    (While we are it, also add the boot ID mount to the existing list, as
    nspawn and other container managers overmount that too, typically, and
    it is thus owned by the container manager and not the payload
    typically.)