Match autocons
Skip unless EFI x86_64.
If SPCR, trust it and use that unconditionally.
Otherwise, if only one can respond to TIOCMGET, then use that one.
If multiple can respond, but exactly one shows carrier, use that.
RedHat makes grub redundantly handle serial output that firmware
already handles. If we detect EFI firmware and SPCR ACPI table and
connected serial port, that suggests that firmware will handle.
Ubuntu hates serial console by default, amend it so it can actually
work for serial users.
Jarrod Johnson