stand/lua: Set reasonable ACPI default based on presence

Set it based on hint.acpi.0.rsdp. Initially, hint.acpi.0.disabled will be
respected. "Using System Defaults" will override whether it's explicitly
disabled by hint and re-load it based on whether it's present on the system.

Unlike the 4th version, this is not restricted to x86. I have no strong
reasoning for this, so this is definitely open to change.