Avoid shell in systemctl helpers

[Asylian21]

Summary

• replace shell-based systemctl status checks with subprocess.run argument lists
• replace the journalctl | head shell pipeline with direct journalctl -n 300 arguments
• keep existing status return-code behavior and service enabled cache behavior

Validation

• git diff --check
• PYTHONPATH=rootfs/standard/var/pynode python3 -m py_compile rootfs/standard/var/pynode/systemctl_info.py
• smoke-tested the command builders with mocked subprocess.run and subprocess.check_output