kernel - Add APIC_IO conf option back in / make it adjust hw.apic_io_enable

kernel - Add APIC_IO conf option back in / make it adjust hw.apic_io_enable

* APIC_IO again controls whether a SMP kernel will use the io-apic or not.
  All it does now is adjust the hw.apic_io_enable default value.

  Changing the code to allow apic-io to be soft-selectable was great, but
  removing the kernel config option was not so great.

* SMP Kernels compiled without APIC_IO again default to not using it.  You
  can use APIC_IO either by configuring the kernel with the option or
  by setting hw.apic_io_enable=1 in /boot/loader.conf.

* SMP Kernels compiled with APIC_IO default to using APIC_IO.

* Reduces confusion between 2.8 and 2.9 kernel config files.

* Reduces personal frustrations.