Inform about new thread in a single place.

    Inform about new thread in a single place.

        * thread.c (add_thread_silent): Renamed
        from add_thread.
        (print_thread_events): New variable definition.
        (show_print_thread_events): New function.
        (_initialize_thread): Add "set print thread-events" and
        "show print thread-events" commands.
        (add_thread): Announce new thread.
        * gdbthread.h (add_thread_silent): Declare.
        (print_thread_events): New variable declaration.
        * inf-ttrace.c (inf_ttrace_wait): Don't
        inform about new thread, as add_thread is always
        called too, and will take care of that.
        * infrun.c (handle_inferior_event): Likewise.
        * procfs.c (procfs_wait): Likewise.
        * remote.c (remote_currthread): Likewise.
        * sol-thread.c (sol_thread_wait): Likewise.
        * win32-nat.c (get_win32_debug_event): Likewise.
        * linux-thread-db.c (attach_thread): Likewise.
        Remove the verbose parameter.
        (check_event): Make detach_thread be verbose
        only if print_thread_events is set.
        * linux-nat.c (lin_lwp_attach_lwp): Don't inform
        about new thread.  This is called only from
        linux-thread-db.c:attach_thread, which will take care.
        Remove the verbose parameter.
        * linux-nat.h (lin_lwp_attach_lwp): Adjust prototype.