Add a new taskqueue setup method that takes a cpuid to pin the
taskqueue worker thread(s) to.

For now it isn't a taskqueue/taskthread error to fail to pin
to the given cpuid.

Thanks to rpaulo@, kib@ and jhb@ for feedback.

Tested:

• igb(4), with local RSS patches to pin taskqueues.

TODO:

• ask the doc team for help in documenting the new API call.
• add a taskqueue_start_threads_cpuset() method which takes a cpuset_t - but this may require a bunch of surgery to bring cpuset_t into scope.