Give every interpreter a command_loop_proc.

Give every interpreter a command_loop_proc.

  https://sourceware.org/ml/gdb-patches/2013-09/msg00179.html

gdb/ChangeLog

        * cli/cli-interp.c (_initialize_cli_interp): Add a
        command_loop_proc to interp_procs.
        * event-top.c (cli_command_loop): Change signature to match
        interp_command_loop_ftype.
        * event-top.h (cli_command_loop): Same.
        * interps.c (interp_new): Require every interpreter to have a
        command_loop_proc.
        (current_interp_command_loop): Just call the command_loop_proc on
        the current interpreter.
        * tui/tui-interp.c (_initialize_tui_interp): Add a
        command_loop_proc to interp_procs.