* structs.def: Add pid field of struct pthread.
* td_ta_thr_iter.c (iterate_thread_list): Take new arg MATCH_PID.
If a thread's pid does not match nor is < 0 while its tid matches
nor is < 0 and equal to -MATCH_PID, ignore it.
* td_thr_validate.c (td_thr_validate): Validate thread's pid/tid.