Edits `GRUB_CMDLINE_LINUX_DEFAULT`. GRUB menu generators normally add this value to regular Linux menu entries, but not to generated recovery or rescue entries.

Use this for options that make normal boots quieter or more convenient, such as `quiet`, `splash`, or console and graphics workarounds that are not needed for recovery boots.

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 existing non-rescue BLS entries.