- Fix pf(4) to build with MAXCPU set to 256. MAXCPU is actually a count,
not a maximum ID value (so it is a cap on mp_ncpus, not mp_maxid).
- Bump MAXCPU on amd64 from 64 to 256. In practice APIC only permits 255
CPUs (IDs 0 through 254). Getting above that limit requires x2APIC.
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