- Allow ioapic_set_{nmi,smi,extint}() to be called multiple times on the
same pin so long as the pin's mode is the same as the mode being
requested.
- Add a notion of bus type for the interrupt associated with interrupt pin.
This is needed so that we can force all EISA interrupts to be active high
in the forthcoming ioapic_config_intr().
- Fix a bug for EISA systems that didn't remap IRQs. This would have broken
EISA systems that tried to disable mixed mode for IRQ 0.