Systemd 209 started setting $WATCHDOG_PID, and sd-daemon watch was
modified to check for this variable. This means that
sd_watchdog_enabled() stopped working with previous versions of
systemd. But sd-event is a public library and API and we must keep it
working even when a program compiled with a newer version of the
libary is used on a system running an older version of the manager.
getenv() and unsetenv() are fairly expensive calls, so optimize
sd_watchdog_enabled() by not calling them when unnecessary.
man: centralize the description of $WATCHDOG_PID and $WATCHDOG_USEC in
the sd_watchdog_enabled manpage. It is better not to repeat the same
stuff in two places.
Also, introduce a new environment variable named $WATCHDOG_PID which
cotnains the PID of the process that is supposed to send the keep-alive
events. This is similar how $LISTEN_FDS and $LISTEN_PID work together,
and protects against confusing processes further down the process tree
due to inherited environment.
