MFC r257854 (discussed with alc@)

  As of r257209, all architectures have defined
  VM_KMEM_SIZE_SCALE.  In other words, every architecture is now
  auto-sizing the kmem arena.  This revision changes kmeminit() so
  that the definition of VM_KMEM_SIZE_SCALE becomes mandatory and
  the definition of VM_KMEM_SIZE becomes optional.

  Replace or eliminate all existing definitions of VM_KMEM_SIZE.
  With auto-sizing enabled, VM_KMEM_SIZE effectively became an
  alternate spelling for VM_KMEM_SIZE_MIN on most architectures.
  Use VM_KMEM_SIZE_MIN for clarity.