This PR adds a standalone Systemd Services and Units module for managing systemd units across system and user scopes.
The module keeps systemd-specific behavior separate from the legacy Bootup and Shutdown module and is implemented as standalone `strict`/`warnings` Perl code rather than depending on its existing init helpers. Those helpers intentionally smooth over multiple init systems, while this module keeps systemd-specific file handling, user-manager behavior, ACL checks, and control operations explicit, scoped, and easier to audit.
It includes:
- Tabbed views for services, timers, sockets, paths, targets, storage, resources, devices, and user units
- Guided creation and editing for common unit types, with contextual fields, validation, and help
- User-scoped unit management with linger support and safe handling of home-directory unit files
- Runtime actions for start, stop, restart, enable, disable, status, logs, properties, dependencies, and system-unit mask/unmask
- Drop-in override inventory plus create, edit, and delete flows
- Manual unit-file editing with daemon reload reminders and actions
- Configurable module behavior, visible tabs, display options, and post-create navigation
- Comprehensive ACL controls for system/user scopes, actions, manual edits, drop-ins, linger, reload, backup, and user filters
- Safe Webmin user support through a scoped safe ACL preset
- Virtualmin integration for granting domain owners access to their own systemd user units
- Tests for unit generation, safety checks, ACL behavior, user-unit handling, backup coverage, and Perl::Critic compatibility
A companion Virtualmin PR adds template integration so domain owners can be granted scoped access to their own systemd user units when this module is installed.
