pid1: make clear that $WATCHDOG_USEC is set for the shutdown binary, noone else

pid1: make clear that $WATCHDOG_USEC is set for the shutdown binary, noone else

We use the $WATCHDOG_USEC variable for two very closely uses: as part of
the sd_watchdog_enabled() protocol for implementing service watchdogs.
And as part of the protocol between the service manager and
systemd-shutdown across the PID 1 execve() transition during shutdown.

Apparently some exitrds tools got confused by the latter use. Let's
address that by setting $WATCHDOG_PID to 1, in accordance to the
sd_watchdog_enabled() protocol to make clear this is only intended for
PID 1 and nothing else.

Replaces: #35135
(cherry picked from commit 4b20ae9a0e914e61d6bac095e5fc9664510ac03e)