systemd-sysext: ensure daemon reloading

[tormath1]

In this PR, we patch systemd-sysext to perform a daemon reload when running merge, unmerge and refresh subcommands. It's possible to skip the reload with --no-reload.

It allows to drop the daemon reload we have in the ensure-sysext.service (flatcar/init#96)

How to use

Current state:

$ sudo systemd-sysext merge
Using extensions 'podman'.
Merged extensions into '/usr'.
$ systemctl cat podman
# /usr/lib/systemd/system/podman.service
[Unit]
Description=Podman API Service
....
$ sudo systemd-sysext unmerge
Unmerged '/usr'.
$ systemctl cat podman
# Warning: podman.service changed on disk, the version systemd has loaded is outdated.
# This output shows the current version of the unit's original fragment and drop-in files.
# If fragments or drop-ins were added or removed, they are not properly reflected in this output.
# Run 'systemctl daemon-reload' to reload units.
Failed to cat /usr/lib/systemd/system/podman.service: No such file or directory

Patched state:

$ sudo systemd-sysext unmerge
Unmerged '/usr'.
$ systemctl cat podman
No files found for podman.service.

Testing done

• Manual testing

Closes: flatcar/Flatcar#984

• Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
• Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.
• CI: http://jenkins.infra.kinvolk.io:8080/job/container/job/packages_all_arches/1706/cldsv/
• Upstream PR: sysext: add EXTENSION_RELOAD_MANAGER metadata systemd/systemd#27404

Last CI is failing because of: flatcar/mantle#429

[tormath1]

Closing this - it was just testing purposes.

[github-actions]

Build action triggered: https://github.com/flatcar/scripts/actions/runs/5422114680