Does anyone know what the rdunique/wrunique unprivileged PALcode
traps are for? Do they, as they appear, allow one value to be stored
per-thread? How are they used in DEC Unix? What kind of performance
hit do I take trapping to PALcode?
I ask because I was thinking of changing the LinuxThreads code to use
this for its thread id, rather than doing some screwy stack pointer
lookup nonsense as is necessary for other platforms.