shutdown: complain if process excluded from killing spree runs of the same rootfs as PID1

src/basic/process-util.h

@@ -70,6 +70,7 @@ int getenv_for_pid(pid_t pid, const char *field, char **_value);
 
 bool pid_is_alive(pid_t pid);
 bool pid_is_unwaited(pid_t pid);
+int pid_from_same_root_fs(pid_t pid);
 
 bool is_main_thread(void);