process-util: introduce pid_is_my_child() helper

No functional changes.

src/basic/process-util.h

@@ -69,6 +69,7 @@ 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_is_my_child(pid_t pid);
 int pid_from_same_root_fs(pid_t pid);
 
 bool is_main_thread(void);