remove xhat
The xhat infrastructure was added to support hardware such as the zulu
graphics card - hardware which had on-board MMUs.  The VM used the xhat code
to keep the CPU's and Zulu's page tables in-sync.  Since the only xhat user
was zulu (which is gone), we can safely remove it simplifying the whole VM
subsystem.
Assorted notes:
- AS_BUSY flag was used solely by xhat

*** 63,73 ****
   * translations that have page structures.  Therefore, it can
   * underestimate the traditional resident set size, eg, if the
   * physical page is present and the hardware translation is missing;
   * and it can overestimate the rss, eg, if there are active
   * translations to a frame buffer with page structs.
!  * Also, it does not take sharing and XHATs into account.
   */
  size_t
  rm_asrss(as)
          register struct as *as;
  {
--- 63,73 ----
   * translations that have page structures.  Therefore, it can
   * underestimate the traditional resident set size, eg, if the
   * physical page is present and the hardware translation is missing;
   * and it can overestimate the rss, eg, if there are active
   * translations to a frame buffer with page structs.
!  * Also, it does not take sharing into account.
   */
  size_t
  rm_asrss(as)
          register struct as *as;
  {