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

@@ -63,11 +63,11 @@
  * 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.
+ * Also, it does not take sharing into account.
  */
 size_t
 rm_asrss(as)
         register struct as *as;
 {