watchdog: Convert to using sbintime_t format

[jhibbits]

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.

Still TODO: documentation updates, probably a lot of cleanup, too.

Test Plan

This was tested in a VM with SW_WATCHDOG configured, and various debug printf()s added, which I'll commit later.

[jhibbits]

Ping, anyone?

[phk]

As the original author of the watchdog interfaces, I have no objections to this change, but I have not reviewed the diff.

[adrian]

sys/sys/watchdog.h

45

is it ok for these to re-use the same ioctl numbers as the existing API?

[jhibbits]

sys/sys/watchdog.h

45

I believe so, because the type is also encoded into the final ioctl number. At least, there's no build errors, and the right cases are used when tested :)