Edits `GRUB_CMDLINE_LINUX`. GRUB menu generators normally add this value to every generated Linux menu entry, including recovery or rescue entries.

Use this for options that must be present whenever Linux boots, such as storage, root filesystem, LVM, filesystem flag, security, mitigation, or `crashkernel` options.

On BLS-based systems, existing boot entries may store options directly in BLS entry files, or reference `$kernelopts`, a GRUB environment value stored in `grubenv`. Webmin does not edit `grubenv` directly; when `grubby` is available, Webmin asks it to apply changes here to all existing BLS entries.