Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Fix watchdog pretimeout.

The original API calls for pow2ns, however the new APIs from
Linux call for seconds.

We need to be able to convert to/from 2^Nns to seconds in both
userland and kernel to fix this and properly compare units.