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Put the KPCRs in high memory, in their own PML4.

See merge request !173

This also removes the need to do another semi-expensive remap when
cloning processes, since the KPCRs (for kernel TLS) are no longer stored
in the user PML4.

Move most of ACPI to userspace

See merge request !175

We may also want to do this with the MADT and the HPET tables, and let
user drivers specify what the tables mean independent of ACPI. That is,
adding an interface for registering new CPUs, and specifying the main
timer IRQ.

Currently, there are some things that need to be set up by userspace
that the kernel previously did. These include telling firmware when the
I/O APIC is used, and most importantly, shutting down the system.

The former is not particularly important, but for the latter I think
that we could implement this using a "shutdown pipe". Essentially it
will be a file that triggers an event shutting down, which would be used
to notify to acpid that the kernel is requesting a shutdown.