kernel - zero pages during idle

kernel - zero pages during idle

Bring in base work by Venkatesh Srinivas:

* vm/vm_zeropage.c and bzeront() code.

Plus make the following adjustments:

* Move the bzeront code from platform/ to cpu/ and add a x86_64 version
  of bzeront().

* Expose the page zeroing rate via sysctl and adjust the default upwards.

* Expose the movnti feature via sysctl (defaults to off).

* Change poll interval from 1 second to 1/10 second.

* Correct a bug where the MP lock was not being initially released.  The
  thread is created with the MP lock held.  This was causing the hysteresis
  check to fail.

* Correct a bug where vm_page_zero_count was being double-incremented
  due to changes in how the zerod page is freed.

* Clean up the code syntax a bit.

Submitted-by: Venkatesh Srinivas <me@endeavour.zapto.org>