rework the sensor task handling to run in the kernels generic workq.
rework the sensor task handling to run in the kernels generic workq.

apologies to art for abusing timeouts so badly. apologies to tedu for
making his head hurt.

tested by and ok tedu@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars