machdep.c fixed the missing early initialization of curpcb, so curpcb
is now always set together with curthread and it cannot be NULL except
before the IDT has been set up (so trap() is unreachable) or after a
memory error. In any case, it was often used without checking.
curcpb shouldn't exist anyway. It doesn't exist for most non-i386 arches.
It just caches curthread->td_pcb in a global. This was a better idea
before it was per-cpu. trap() and some other places can get at it more
efficiently using td->td_pcb instead of PCPU_GET(curpcb). The main
exception is support.s which mostly wants only curpcb->pcb_onfault.
ab2bf9ff6a