libthread_db thread handles

[Roland McGrath]

> If you have the pthread_t value computing the td_thrhandle_t is an
> operation which can be performed entirely without the looking at the
> inferior.  At least in the new implementation.  Just call
> td_ta_map_id2thr().  This shouldn't add any measurable overhead.
> 
> I would prefer you caching the pthread_t value very much over caching
> any opaque data structure.  If this means adding a function
> td_ta_map_thr2id() I'd have no problems with it.  But not even this
> should be necessary since for both events, TD_CREATE and TD_DEATH, the
> eventdata is the pthread_t value.  And this should be a documented
> interface.

One can already use td_thr_get_info and ti_tid is the pthread_t value (that
can be given to td_ta_map_id2thr).