Tru64: Dead threads are never deleted.

Tru64: Dead threads are never deleted.

        * dec-thread.c (dec_thread_ptid_is_alive): New function.
        (dec_thread_count_gdb_threads): Fix counter increment.
        (dec_thread_add_gdb_thread): Fix *listp increment.
        (resync_thread_list): Fix bug in deletion of dead threads that
        caused all threads to be deleted, instead of just the dead ones.