Use SD_ACTIVATION env. var. in systemd units to better detect systemd in use.

Use SD_ACTIVATION env. var. in systemd units to better detect systemd in use.

LISTEN_PID and LISTEN_FDS environment variables are defined only during systemd
"start" action. But we still need to know whether we're activated during
"reload" action as well - we use the reload action to call "dmeventd -R"/"lvmetad -R"
for statefull daemon restart. We can't use normal "restart" as that is simply
composed of "stop" and "start" and we would lose any state the daemon has.