Rework the interrupt code and add support for interrupt filtering

(INTR_FILTER). This includes:
o  Save a pointer to the sapic structure and IRQ for every vector,
   so that we can quickly EOI, mask and unmask the interrupt.
o  Add locking to the sapic code now that we can reprogram a
   sapic on multiple CPUs at the same time.
o  Use u_int for the vector and IRQ. We only have 256 vectors, so
   using a 64-bit type for it is rather excessive.
o  Properly handle concurrent registration of a handler for the
   same vector.

Since vectors have a corresponding priority, we should not map
IRQs to vectors in a linear fashion, but rather pick a vector
that has a priority in line with the interrupt type. This is left
for later. The vector/IRQ interchange has been untangled as much
as possible to make this easier.

Approved by: re (blacket)

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