(code_convert_region_unwind): ARG is changed to a cons.

(code_convert_region_unwind): ARG is changed to a cons.
(code_convert_region): Adjusted for the above change.
(set_conversion_work_buffer): If the work buffer is already in
use, generate a new buffer and return it.  Otherwise return Qnil.
(run_pre_post_conversion_on_str): Adjusted for the above change.
(run_pre_write_conversin_on_c_str): Likewise.