Load indirect array on demand and flush only on iupdate.

These safety policies help ensure that an accessor does not migrate
CPUs while accessing a percpu variable.  The "cli" safety policy only
allows percpu variables to be accessed when interrupts are disabled.
The "internal" safety policy leaves it up to the caller.  Not
surprisingly, most current percpu variables use internal.

Like the find method of standard containers, this returns an iterator
to the requested key.

I'm sure it fails for some other naming format..

An xns must fit in a page, but that doesn't allow enough buckets
to avoid physical sharing in dirbench.  This is basically a
work-around for the one page limit.

I'll be changing the dirns implementation soon.

..next step is a namex that takes no refs.

The only unexpected sharing is on the VMA refcount, which is
exactly what we expected.

This eliminates a bunch of confusing scopes that come from thread creation

With the mtrace QEMU, which now switches VCPUs when there's a PAUSE,
this speeds up my lame test (usertests vmoverlap) by a factor of 7 and
certainly makes xv6 feel much faster.