(code_convert_string): Add record_unwind_protect to

(code_convert_string): Add record_unwind_protect to
assure setting inhibit_pre_post_conversion back to zero.  Take
care of the multibyteness of the working buffer.

(inhibit_pre_post_conversion): New variable.
(setup_coding_system): If inhibit_pre_post_conversion is nonzero,
ignore post-read-conversion and pre-write-conversion property of
the coding system.
(code_convert_region_unwind): New function.
(code_convert_region): Set inhibit_pre_post_conversion to 1 while
running pre-write-conversion and post-read-conversion.
(code_convert_string): Likewise.