Impact: Reverts default reboot method.

Checkin 14d7ca5c changed the default
reboot method to "pci", a.k.a. port CF9.  Unfortunately this has been
shown to cause lockups on at least two systems for which REBOOT_KBD
worked, both Thinkpads with Intel chipsets.  This reverts the default
to REBOOT_KBD, while leaving the option to have "reboot=pci" specified
explicitly or via a DMI match.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>