socket: support setting ownership of message queues

This applies the existing SocketUser=/SocketGroup= options to units
defining a POSIX message queue, bringing them in line with UNIX
sockets and FIFOs.  They are set on the file descriptor rather than
a file system path because the /dev/mqueue path interface is an
optional mount unit.

test/units/TEST-07-PID1.mqueue-ownership.sh

@@ -0,0 +1,46 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: LGPL-2.1-or-later
+set -eux
+set -o pipefail
+
+# Verify ownership attributes are applied to message queues
+
+# Select arbitrary non-default attributes to apply to the queue.
+queue=/attr_q  # Pick any unused queue name.
+user=nobody  # Choose a core system user.
+group=adm  # Choose a core system group.
+mode=0420  # Allow the owner to read messages and anyone in the group to write.
+
+at_exit() {
+    set +e
+    systemctl stop mqueue-ownership.{service,socket}
+    rm -f /run/systemd/system/mqueue-ownership.{service,socket}
+    systemctl daemon-reload
+}
+trap at_exit EXIT
+
+cat << EOF > /run/systemd/system/mqueue-ownership.socket
+[Unit]
+Description=Create a message queue with customized ownership
+[Socket]
+ListenMessageQueue=/${queue#/}
+RemoveOnStop=true
+SocketUser=$user
+SocketGroup=$group
+SocketMode=$mode
+EOF
+
+cat << 'EOF' > /run/systemd/system/mqueue-ownership.service
+[Unit]
+Description=Dummy service for the socket unit
+Requires=%N.socket
+[Service]
+ExecStart=/usr/bin/true
+Type=oneshot
+EOF
+
+systemctl daemon-reload
+systemctl start mqueue-ownership.socket
+
+systemctl start dev-mqueue.mount  # Ensure this file path interface is mounted.
+[[ $(stat -c '%04a %U %G' "/dev/mqueue/${queue#/}") == "$mode $user $group" ]]