process-util: port pid_from_same_root_fs() to pidref, and port three places over to it

src/basic/process-util.h

@@ -93,7 +93,7 @@ int pid_is_unwaited(pid_t pid);
 int pidref_is_unwaited(const PidRef *pidref);
 int pid_is_my_child(pid_t pid);
 int pidref_is_my_child(const PidRef *pidref);
-int pid_from_same_root_fs(pid_t pid);
+int pidref_from_same_root_fs(PidRef *a, PidRef *b);
 
 bool is_main_thread(void);