nspawn: move $NOTIFY_SOCKET into /run/host/ too

nspawn: move $NOTIFY_SOCKET into /run/host/ too

The sd_notify() socket that nspawn binds that the payload can use to
talk to it was previously stored in /run/systemd/nspawn/notify, which is
weird (as in the previous commit) since this makes /run/systemd
something that is cooperatively maintained by systemd inside the
container and nspawn outside of it.

We now have a better place where container managers can put the stuff
they want to pass to the payload: /run/host/, hence let's make use of
that.

This is not a compat breakage, since the sd_notify() protocol is based
on the $NOTIFY_SOCKET env var, where we place the new socket path.