Revert "cgroup: do not create a sub-cgroup by default"

[giuseppe]

This reverts commit 262d6ac.

This is a breaking change, we need a version bump.

Summary by Sourcery

Revert the change that disabled automatic creation of a systemd sub-cgroup by default, restoring the previous behavior where a "container" subgroup is generated if no custom subgroup is provided.

Enhancements:

• Restore default creation of the systemd sub-cgroup for containers
• Return "container" as the default subgroup name when none is specified in code
• Update documentation to reflect that the default subgroup is "container" on cgroup v2 and "" on cgroup v1

[sourcery-ai]

Reviewer's Guide

This PR reverts the prior change that disabled default systemd sub-cgroup creation by restoring the "container" fallback in code and aligning the manpage and markdown documentation with the revived default behavior, necessitating a version bump.

File-Level Changes

Change	Details	Files
Restore default systemd sub-cgroup creation logic	Change find_systemd_subgroup to return "container" instead of NULL Reinstate default fallback behavior for unspecified SUBGROUP	src/libcrun/cgroup-systemd.c
Update documentation for run.oci.systemd.subgroup semantics	Revise description to reflect override-only behavior Clarify default values: "container" on cgroup v2, "" on cgroup v1 Adjust example paths to match restored defaults	crun.1 crun.1.md

Possibly linked issues

• Travis integration #1: The PR reverts a change that altered default systemd sub-cgroup creation in crun, resolving the reported regression where crun fails in systemd services due to RestrictAddressFamilies.

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[sourcery-ai]

Hey there - I've reviewed your changes - here's some feedback:

• Don’t forget to bump the project version (e.g. in configure.ac or NEWS) since this revert is a breaking change.
• The code now always returns "container"—you should detect cgroup v1 vs v2 and return an empty string for v1 to match the docs.
• The example path /sys/fs/cgroup//system.slice/... has a double slash and looks confusing—consider fixing it to show the intended layout.

Prompt for AI Agents

Please address the comments from this code review:
## Overall Comments
- Don’t forget to bump the project version (e.g. in configure.ac or NEWS) since this revert is a breaking change.
- The code now always returns "container"—you should detect cgroup v1 vs v2 and return an empty string for v1 to match the docs.
- The example path `/sys/fs/cgroup//system.slice/...` has a double slash and looks confusing—consider fixing it to show the intended layout.

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[packit-as-a-service]

TMT tests failed. @containers/packit-build please check.

[flouthoc]

LGTM