ow(4): protocol timings can now be changed as sysctl-s / tunables

I limited potentially infinite timings by 960 us based on a footnote on
page 38 of Maxim Integrated Application Note 937, Book of iButton
Standards: "In order not to mask interrupt signalling by other devices
on the 1–Wire bus, tRSTL + tR should always be less than 960 us."

MFC after: 3 weeks