Really slam the door on mixed mode now that we don't depend on it for a

Really slam the door on mixed mode now that we don't depend on it for a
working IRQ0 with APIC anymore.  Previously, it was possible to have
some other ATPIC IRQS "leak" through in a few edge cases.  For example, on
my x86 test machine, ACPI re-routes the SCI (IRQ 9) to intpin 13 on the
first I/O APIC.  This leaves a hole for IRQ 13 (since the APIC doesn't
provide a source for IRQ 13 in that case) with the result that the ATPIC
IRQ13 source was registered instead.  This changes the 8259A drivers to
only register their interrupt sources if none of the 16 ISA IRQs have an
interrupt source already installed.

MFC after:	1 week