Fix many bugs and issues in the VM system, particularly related to

Fix many bugs and issues in the VM system, particularly related to
heavy paging.

* (cleanup) PG_WRITEABLE is now set by the low level pmap code and not by
  high level code.  It means 'This page may contain a managed page table
  mapping which is writeable', meaning that hardware can dirty the page
  at any time.  The page must be tested via appropriate pmap calls before
  being disposed of.

* (cleanup) PG_MAPPED is now handled by the low level pmap code and only
  applies to managed mappings.  There is still a bit of cruft left over
  related to the pmap code's page table pages but the high level code is now
  clean.

* (bug) Various XIO, SFBUF, and MSFBUF routines which bypass normal paging
  operations were not properly dirtying pages when the caller intended
  to write to them.

* (bug) vfs_busy_pages in kern/vfs_bio.c had a busy race.  Separate the code
  out to ensure that we have marked all the pages as undergoing IO before we
  call vm_page_protect().  vm_page_protect(... VM_PROT_NONE) can block
  under very heavy paging conditions and if the pages haven't been marked
  for IO that could blow up the code.

* (optimization) Make a minor optimization.  When busying pages for write
  IO, downgrade the page table mappings to read-only instead of removing
  them entirely.

* (bug) In platform/pc32/i386/pmap.c fix various places where
  pmap_inval_add() was being called at the wrong point.  Only one was
  critical, in pmap_enter(), where pmap_inval_add() was being called so far
  away from the pmap entry being modified that it could wind up being flushed
  out prior to the modification, breaking the cpusync required.

  pmap.c also contains most of the work involved in the PG_MAPPED and
  PG_WRITEABLE changes.

* (bug) Close numerous pte updating races with hardware setting the
  modified bit.  There is still one race left (in pmap_enter()).

* (bug) Disable pmap_copy() entirely.   Fix most of the bugs anyway, but
  there is still one left in the handling of the srcmpte variable.

* (cleanup) Change vm_page_dirty() from an inline to a real procedure, and
  move the code which set the object to writeable/maybedirty into
  vm_page_dirty().

* (bug) Calls to vm_page_protect(... VM_PROT_NONE) can block.  Fix all cases
  where this call was made with a non-busied page.  All such calls are
  now made with a busied page, preventing blocking races from re-dirtying
  or remapping the page unexpectedly.

  (Such blockages could only occur during heavy paging activity where the
  underlying page table pages are being actively recycled).

* (bug) Fix the pageout code to properly mark pages as undergoing I/O before
  changing their protection bits.

* (bug) Busy pages undergoing zeroing or partial zeroing in the vnode pager
  (vm/vnode_pager.c) to avoid unexpected effects.