-Wwrite-strings: Constify mi_cmd_argv_ftype's 'command' parameter

-Wwrite-strings: Constify mi_cmd_argv_ftype's 'command' parameter

-Wwrite-strings flags this attempt to pass a literal to a "char *":

	mi_cmd_interpreter_exec ("-interpreter-exec", argv, 2);

Fix that by constifying mi_cmd_argv_ftype's 'command' parameter and
adjusting all MI commands.

gdb/ChangeLog:
2017-04-05  Pedro Alves  <palves@redhat.com>

	* mi/mi-cmd-break.c (mi_cmd_break_insert_1, mi_cmd_break_insert)
	(mi_cmd_dprintf_insert, mi_cmd_break_passcount)
	(mi_cmd_break_watch, mi_cmd_break_commands): Constify 'command'
	parameter.
	* mi/mi-cmd-catch.c (mi_cmd_catch_assert, mi_cmd_catch_exception)
	(mi_cmd_catch_load, mi_cmd_catch_unload): Constify cmd' parameter.
	* mi/mi-cmd-disas.c (mi_cmd_disassemble): Constify 'command'
	parameter.
	* mi/mi-cmd-env.c (mi_cmd_env_pwd, mi_cmd_env_cd, mi_cmd_env_path)
	(mi_cmd_env_dir, mi_cmd_inferior_tty_set, _cmd_inferior_tty_show)
	* mi/mi-cmd-file.c (mi_cmd_file_list_exec_source_file)
	(mi_cmd_file_list_exec_source_files)
	(mi_cmd_file_list_shared_libraries): Constify 'command' parameter.
	* mi/mi-cmd-info.c (mi_cmd_info_ada_exceptions)
	(mi_cmd_info_gdb_mi_command, mi_cmd_info_os): Constify 'command'
	parameter.
	* mi/mi-cmd-stack.c (mi_cmd_enable_frame_filters)
	(mi_cmd_stack_list_frames, mi_cmd_stack_info_depth)
	(mi_cmd_stack_list_locals, mi_cmd_stack_list_args)
	(mi_cmd_stack_list_variables, mi_cmd_stack_select_frame)
	(mi_cmd_stack_info_frame): Constify 'command' parameter.
	* mi/mi-cmd-target.c (mi_cmd_target_file_get)
	(mi_cmd_target_file_put, mi_cmd_target_file_delete): Constify
	'command' parameter.
	* mi/mi-cmd-var.c (mi_cmd_var_create, mi_cmd_var_delete)
	(mi_cmd_var_set_format, mi_cmd_var_set_visualizer)
	(mi_cmd_var_set_frozen, mi_cmd_var_show_format)
	(mi_cmd_var_info_num_children, mi_cmd_var_list_children)
	(mi_cmd_var_info_type, mi_cmd_var_info_path_expression)
	(mi_cmd_var_info_expression, mi_cmd_var_show_attributes)
	(mi_cmd_var_evaluate_expression, mi_cmd_var_assign)
	(mi_cmd_var_update, mi_cmd_enable_pretty_printing)
	(mi_cmd_var_set_update_range): Constify 'command' parameter.
	* mi/mi-cmds.h (mi_cmd_argv_ftype): Constify 'command' parameter.
	* mi/mi-interp.c (mi_cmd_interpreter_exec): Constify 'command'
	parameter.
	* mi/mi-main.c (mi_cmd_gdb_exit, mi_cmd_exec_next)
	(mi_cmd_exec_next_instruction, mi_cmd_exec_step)
	(mi_cmd_exec_step_instruction, mi_cmd_exec_finish)
	(mi_cmd_exec_return ,mi_cmd_exec_jump, mi_cmd_exec_continue)
	(mi_cmd_exec_interrupt, mi_cmd_exec_run, mi_cmd_target_detach)
	(mi_cmd_target_flash_erase, mi_cmd_thread_select)
	(mi_cmd_thread_list_ids, mi_cmd_thread_info)
	(mi_cmd_list_thread_groups, mi_cmd_data_list_register_names)
	(mi_cmd_data_list_changed_registers)
	(mi_cmd_data_write_register_values)
	(mi_cmd_data_evaluate_expression, mi_cmd_data_read_memory)
	(mi_cmd_data_read_memory_bytes, mi_cmd_data_write_memory)
	(mi_cmd_data_write_memory_bytes, mi_cmd_enable_timings)
	(mi_cmd_list_features, mi_cmd_list_target_features)
	(mi_cmd_add_inferior, mi_cmd_remove_inferior)
	(mi_cmd_trace_define_variable, mi_cmd_trace_list_variables)
	(mi_cmd_trace_find, mi_cmd_trace_save, mi_cmd_trace_start)
	(mi_cmd_trace_status, mi_cmd_trace_stop, mi_cmd_ada_task_info)
	(mi_cmd_trace_frame_collected): Constify 'command'
	parameter.
	* mi/mi-symbol-cmds.c (mi_cmd_symbol_list_lines): Constify
	'command' parameter.