Implement an optimization for INTREN/INTRDIS that bde pointed out last

time I tinkered around here.  Since INTREN is called from the interrupt
critical path now, it should not be too expensive.  In this case, we
look at the bits being changed to decide which 8 bit IO port to write to
rather than unconditionally writing to both.  I could probably have gone
further and only done the write if the bits actually changed, but that
seemed overkill for the usual case in interrupt threads.

[an outb is rather expensive when it has to cross the ISA bus]

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