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.

So we can hush up spurious errors.

The user errors stream is disabled by default.

These have the type-safety and extensibility of ostreams, but the
simplicity and almost the terseness of printf.

This might be a mistake because we can't control the order of global
construction, but we already have several global constructors that
were simply not being called before.  As long as we keep all global
constructors simple, this should be okay.

This fixes the deadlock where a thread tries to allocate an inode,
and calls ilock, while another thread is simultaneously freeing
the same inode.

Previously we assumed in many places that all pages between a VMA's
vma_start and vma_end belonged to that VMA.  For radix, the radix tree
is now the source of truth about which pages a VMA maps.  This will
let us replace subranges of a VMA without having to split the VMA into
two ranges (which would violate the commutativity rule; e.g. for
disjoint unmaps within the same VMA).

These methods are to support changes in the VM system.  key() and
span() let you interrogate the current position of the iterator.
next_change() and the copy/move constructors make it easy to work with
spans of equal elements.

I think the semantics used to be that the iterator would always start
at the first non-null element, which I broke when I switched to the
iterative implementation.  This fixes that and introduces a method
that will be useful once iterators can point to null elements.

This puts the trivial part of the radix_iterator constructor used by
end() in the class definition, so the compiler can see the full
definition of end().  This should let it inline it and, hopefully,
constant-eliminate it so it doesn't get called repeatedly.