Remove cleanups from prepare_execute_command

This changes prepare_execute_command to return a scoped_value_mark
rather than a cleanup.

ChangeLog
2017-10-19  Tom Tromey  <tom@tromey.com>

	* mi/mi-main.c (mi_cmd_execute): Update.
	* top.h (prepare_execute_command): Return scoped_value_mark.
	* value.h (class scoped_value_mark): Use DISABLE_COPY_AND_ASSIGN.
	Add move constructor.
	* top.c (prepare_execute_command): Return scoped_value_mark.
	(execute_command): Update.