watchdog: Allow the watchdog to be disabled at runtime

watchdog: Allow the watchdog to be disabled at runtime

manager_{get|set|override}_watchdog check the validity of the new
timeout or the overridden timeout values using timestamp_is_set which
does not recognize "0" as a valid value. However since f16890f, "0"
indicates a disabled watchdog and so is a value we should be able to
configure in order to disable the watchdog. A value of USEC_INFINITY is
considered a no-op. The behavior should be the same for all watchdog
timeout configurations (runtime, pretimeout, and shutdown).