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.)

(cherry picked from commit 6f997852)