This PR expands the "Bootup and Shutdown" module’s systemd support with creation and management for services, timers, sockets, paths and targets. It also adds user-scoped systemd units, linger controls, status and log actions, tabbed unit listings by type, and contextual help for the new options.
For user-scoped units, the implementation includes several safety guards because unit files live under user-controlled home directories:
- User accounts are validated with system account data before any user-unit operation is attempted.
- User unit names are restricted to known systemd unit suffixes and safe filename characters.
- User unit files are limited to direct children of `~/.config/systemd/user`.
- Symlinked `.config`, `.config/systemd`, and `.config/systemd/user` paths are rejected.
- User unit reads, writes, directory creation, and deletes are performed after dropping privileges to the target Unix user.
- File operations re-check paths close to the actual read/write/delete operation to reduce symlink race exposure.
- User unit create failures roll back half-created files when daemon reload fails.
- User-provided unit names, owners, paths, command output, and logs are HTML-escaped before display.
- systemctl, journalctl, and loginctl command arguments are shell-quoted before execution.
- User services omit `User=` and `Group=` directives because they already run under the selected user’s systemd manager.
Together, all these changes will allow Webmin admin to manage both system and user systemd units while keeping user-controlled home-directory paths from becoming root-level file read/write/delete exploits.
Implemented in response to these two issue requests https://github.com/webmin/webmin/issues/2733 and https://github.com/webmin/webmin/issues/2734
