The attached patch fixes a problem in gdb when a corefile is read in
after a multithreaded application has been debugged.  What happens is that
the thread-db and lin-lwp layers are still around and run into internal
errors.

The solution is simply to unpush the thread-db ops in its mourn_inferior
routine.  If a corefile gets loaded, there is no thread-db to interfere.
If another multi-threaded app gets loaded, the thread_db_new_objfile is
designed to bring back the thread-db layer as needed.

This fix solves another failure in the killed.exp testsuite as well.

Ok to commit?

-- Jeff J.