Memwalks run CBs on all PTEs, not just PTE_Ps

The PTE states need a lot of work, and something is likely to be screwed
up here.

I needed to make a distinction between !PTE_P and completely empty,
since the upcoming page map removals will have PTEs !P, but still
'readable'.  And I'll need to memwalk those.
x86: Fixes bug in pml callbacks

Was leaking internal page tables for every process.
Page flags are now atomic

I don't want to lock pages just to mark it dirty or something.  This
will also help the PM removal.
Fixes memory leak in dev stdout

Failed to free the strdup buffer.
Fixed memory leak in namec()

Calling nexterror() instead of error() popped off the second waserror,
skipping over a bunch of cleanup.  Careful with nexterror()!
Fixes waserror() memory leak in devwalk

The alloc was being ignored by the compiler.  As far as it was
concerned, at the waserror point (where error jumps back to), there was
no way for alloc to not be 0.  So we never freed the chan wq->clone.

We'll definitely have issues like this in the future.  We need some way
to tell the compiler to forget anything it might have known before a
certain point; otherwise we'll need to drop lots of volatiles around.
Not sure why we need this but plan9 didn't.
Fixes memory leak in namec()

I neglected to free e.off when skipping the namelenerror.
Page map interface and munmap changes

The PM interface was slimmed a bit.  We still will need to change the
usages of pm_put_page for mmap a little.

munmap is a bit more efficient, and the VMRs are explicitly unmapped.
We'll need to do this instead of just blindly decreffing an entire
address space, since we need to handle PM pages differently.
Splits the mm_lock

Split into VMR (protects VMR tree properties) and PTE (protects page
table structures and properties).

Needed so that we can have the PM lock holders change the PTEs.
Ordering is now: vmr -> pte, vmr -> pm -> pte.
VMRs that map page_maps are tracked

Needed so we can remove pages or otherwise see which processes are using
which parts of a PM.