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.

test/units/testsuite-13.sh

@@ -60,7 +60,7 @@ function check_norbind {
 
 function check_notification_socket {
     # https://github.com/systemd/systemd/issues/4944
-    local _cmd='echo a | $(busybox which nc) -U -u -w 1 /run/systemd/nspawn/notify'
+    local _cmd='echo a | $(busybox which nc) -U -u -w 1 /run/host/notify'
     # /testsuite-13.nc-container is prepared by test.sh
     systemd-nspawn $SUSE_OPTS--register=no -D /testsuite-13.nc-container /bin/sh -x -c "$_cmd"
     systemd-nspawn $SUSE_OPTS--register=no -D /testsuite-13.nc-container -U /bin/sh -x -c "$_cmd"