non-stop and current thread exiting

[Pedro Alves]

Guys,

 Sorry for leaving the discussion for so long.  I was experimenting
with the options, so I could get a larger picture of what was implied.

I've implemented and tried the several options, and ended up settling with
the c) option.  That is, if the selected thread exits, GDB will have no 
selected thread; most commands then error out until the user/frontend
selects a thread.  Already exited threads may be left in the
internal thread list while the core still needs them, but are not
visible to the user.

Since all-stop doesn't accept commands while the inferior is running,
and switches to the thread that gets a stop event automatically, 
it is unaffected.  I propose we go with this behaviour first, and
let people experiment with it.

I'm going to post this as one of the topmost patches of a new
non-stop series, now that all it's dependencies have
been checked in.

-- 
Pedro Alves