watchdog: Convert to using sbintime_t format

Summary:
Some watchdogs are now based on a countdown timer instead of a bit
check. To deal with these, convert the watchdog framework to use
sbintime_t instead of power-of-2-nanoseconds. This allows more
precision, and more variety of watchdog timeouts. Keep the old method
as a compatibility layer, so that drivers can be migrated slowly, as
needed.

Reviewed by: jhb
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D49183