Various APIC/ACPI fixes for x86-64. This brings us closer to working
out of the box on the now popular VIA and NVidia Nforce3 based Athlon64
and Opteron boards. To be really good we would need more ACPI changes
(still waiting for that to be all merged through the usual channels).

With this we mostly work with acpi=off at least.

Also it syncs us up with bugfixes done in 2.4.

 - Disable IO-APIC by default on non SMP VIA/NVidia boards.  This is a
   bit of a hack, but needed to work around ACPI bugs.  Can be
   overwriten with "apic". 
 - Add acpi=ht, meaning run ACPI boot setup, but do not enable the
   interpreter.  Same as i386.
 - Stop MADT parsing early when local APIC or IO-APIC are disabled
 - Add more option parsing early enough to actually change the boot
   process
 - Update documentation for command line options