set minimum darwin kernel rx sleep

set minimum darwin kernel rx sleep

never be willing to sleep less than 500ms. additional tweaks to the
rx scheduler may be needed however before we always slept for 500; now
we'll sleep longer if no events are pending (but not forever)

Change-Id: I7fdb02dd7a383131fffcfae532d71c8fbeb97545
Reviewed-on: http://gerrit.openafs.org/1290
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>