Installing/Upgrading multipath-tools leads to multipathd.socket being inactive

This is an issue with the systemd service not stating the dependency on the socket, but stating a start order with the socket. The fix here is to add

Wants=multipathd.socket

in multipathd.service which is a weak dependency, so the chance of issues arising from it is low, but helps to solve this particular issue.

[Impact]

On Upgrade/Install, the multipathd.socket states that it is inactive. An error is seen during install/upgrade that says

Job failed. See "journalctl -xe" for details.

This issue affects the following packages:

Lunar (0.8.8-1ubuntu2.1)
Jammy (0.8.8-1ubuntu1.22.04.2)
Focal (0.8.3-1ubuntu2.2)
- On upgrade
- On first install

Mantic (0.9.4-5ubuntu3)
- On first install

The issue is because of the order of the systemctl command and the systemd unit files.

multipathd.service has:
...
After=multipathd.socket
...
Also=multipathd.socket

multipath-tools.postinst has:
Jammy/Lunar/Focal (added by dh_systemd_start)
deb-systemd-invoke start 'multipathd.service' 'multipathd.socket' >/dev/null || true

Mantic (added by dh_installsystemd)
if [ -n "$2" ]; then
    _dh_action=restart
else
    _dh_action=start
fi
deb-systemd-invoke $_dh_action 'multipathd.service' 'multipathd.socket' >/dev/null || true

In other words,
systemctl stop multipathd.service multipathd.socket
systemctl start multipathd.service multipathd.socket

will fail

systemctl stop multipathd.service multipathd.socket
systemctl start multipathd.socket multipathd.service

will succeed

If we try to start the socket after the service, we get a failure. This is by design so multiple processes of the socket are not active at once.

[Test Cases]
Due to the nature of this bug/fix, multiple test cases are defined here:
1. Install multipath-tools when it is uninstalled
2. Upgrade multipath-tools to this version
3. Downgrade multipath-tools to this version
4. Reinstall multipath-tools when it is this version
5.
systemctl stop multipathd.service multipathd.socket
systemctl start multipathd.service multipathd.socket

All test cases should result in _no errors_ and both multipathd.service multipathd.socket are active

[Reproducer]
$ lxc launch ubuntu-daily:mantic m --vm
$ lxc shell m
# apt purge -y multipath-tools
# apt update -y
# apt install -y multipath-tools
# systemctl status multipathd.service multipathd.socket | grep Active -B2
● multipathd.service - Device-Mapper Multipath Device Controller
     Loaded: loaded (/lib/systemd/system/multipathd.service; enabled; preset: enabled)
     Active: active (running) since Wed 2023-09-13 19:30:56 UTC; 47s ago
--
○ multipathd.socket - multipathd control socket
     Loaded: loaded (/lib/systemd/system/multipathd.socket; enabled; preset: enabled)
     Active: inactive (dead)