watchdog: clarify that we set the *watchdog* timeout

watchdog: clarify that we set the *watchdog* timeout

This makes sure we mention the word "watchdog" in every log message
related to the watchdog.

Also, this uses the expression "hardware timeout" when referring to the
primary timeout of the watchdog, as opposed to the "pretimeout".

(Not ideal wording I know, but it's preexisting to some point, I just
continued it. I think it's OK though, in particular to underline the
difference to the software watchdog logic we implement via WATCHDOG= in
sd_notify().)

Fixes: #31662