Here's the rewrite of this patch to use cleanups and refcounting on the
thread_info objects to prevent them from being deleted while the thread
is held in a cleanup.  This version completelly drops the
external/internal thread separation, and sticks with inferior_ptid
to mean current thread.  Exited threads are kept in the thread list
until safe to delete.  Most commands aren't allowed while the
user has an exited thread selected.

Tested on x86_64-unknown-linux-gnu -m32, sync/async.

-- 
Pedro Alves