systemd: add env variable SYSTEMD_INSTANCE

systemd/nosocket/kresd@.service.in

@@ -9,6 +9,7 @@ After=network-online.target
 
 [Service]
 Type=notify
+Environment="SYSTEMD_INSTANCE=%i"
 WorkingDirectory=@systemd_work_dir@
 ExecStart=@sbin_dir@/kresd --config=@etc_dir@/kresd.conf --forks=1
 User=@user@