kernel - Refactor Xinvltlb (3)

kernel - Refactor Xinvltlb (3)

* Rollup invalidation operations for numerous kernel-related pmap, reducing
  the number of IPIs needed (particularly for buffer cache operations).

* Implement semi-synchronous command execution, where target cpus do not
  need to wait for the originating cpu to execute a command.  This is used
  for the above rollups when the related kernel memory is known to be accessed
  concurrently with the pmap operations.

* Support invalidation of VA ranges.

* Support reduction of target cpu set for semi-synchronous commands, including
  invltlb's, by removing idle cpus from the set when possible.