Restore keymap option validation and GRUB build-all safeguards

[assisted-by-ai]

Summary

• enforce command-specific option validation and build-all argument checks across keymap scripts and document them in the test plan
• allow GRUB build-all to read layouts from stdin, guard against empty lists, restore nullglob state, and skip update-grub when requested
• refresh GRUB and labwc help text to reflect supported options

Testing

• not run (not requested)

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Codex Task