core: Bind mount notify socket to /run/host/notify in sandboxed units

To be able to run systemd in a Type=notify transient unit, the notify
socket can't be bind mounted to /run/systemd/notify as systemd in the
transient unit wants to use that as its own notify socket which conflicts
with systemd on the host.

Instead, for sandboxed units, let's bind mount the notify socket to
/run/host/notify as documented in the container interface. Since we don't
guarantee a stable location for the notify socket and insist users use
$NOTIFY_SOCKET to get its path, this is safe to do.

test/units/TEST-50-DISSECT.dissect.sh

@@ -2,7 +2,7 @@
 # SPDX-License-Identifier: LGPL-2.1-or-later
 # -*- mode: shell-script; indent-tabs-mode: nil; sh-basic-offset: 4; -*-
 # ex: ts=8 sw=4 sts=4 et filetype=sh
-# shellcheck disable=SC2233,SC2235
+# shellcheck disable=SC2233,SC2235,SC2016
 set -eux
 set -o pipefail
 
@@ -79,6 +79,11 @@ systemd-run --wait -p RootImage="$MINIMAL_IMAGE.raw" mountpoint /run/systemd/jou
 (! systemd-run --wait -p RootImage="$MINIMAL_IMAGE.raw" -p BindLogSockets=no ls /run/systemd/journal/socket)
 (! systemd-run --wait -p RootImage="$MINIMAL_IMAGE.raw" -p MountAPIVFS=no ls /run/systemd/journal/socket)
 
+# Test that the notify socket is bind mounted to /run/host/notify in sandboxed environments and
+# $NOTIFY_SOCKET is set correctly.
+systemd-run --wait -p RootImage="$MINIMAL_IMAGE.raw" -p NotifyAccess=all --service-type=notify --pipe sh -c 'echo READY=1 | ncat --unixsock --udp $NOTIFY_SOCKET --source /run/notify && ls /run/host/notify'
+systemd-run --wait -p RootImage="$MINIMAL_IMAGE.raw" -p NotifyAccess=all --service-type=notify --pipe sh -c 'echo READY=1 | ncat --unixsock --udp $NOTIFY_SOCKET --source /run/notify && env' | grep NOTIFY_SOCKET=/run/host/notify
+
 systemd-run -P -p RootImage="$MINIMAL_IMAGE.raw" cat /usr/lib/os-release | grep -q -F "MARKER=1"
 mv "$MINIMAL_IMAGE.verity" "$MINIMAL_IMAGE.fooverity"
 mv "$MINIMAL_IMAGE.roothash" "$MINIMAL_IMAGE.foohash"