kernel - Refactor swapoff scan code

* Refactor the scan code to be far more optimal.  The old scan was
  restarting the entire VM object scan on each swap block removed
  from the swap device being turned off.

* Scan twice before giving up to try to catch any elements which
  are in-transition.
kernel - Improve vm.vmmeter sysctl performance

* Improve performance by reducing the rate at which the marker is moved.

* Fix a bug, noting that lwkt_yield() can cause us to lose the token,
  by always using the marker for both it and the collision case.
kernel/arpresolve: Initialize 'rt' with NULL.

This fixes a case where NULL was passed for 'rt0' and 'rt' could end up
uninitialized at the log() call in line 509.

Review by sephe showed that we never pass NULL as 'rt0' but let's fix it
anyway, to be safer.

Reported-by: enroljas
Reviewed-by: sephe