(MINIMUM_CONVERSION_BUFFER_SIZE): Macro deleted.

(MINIMUM_CONVERSION_BUFFER_SIZE): Macro deleted.
(conversion_buffer, conversion_buffer_size): Variables deleted.
(get_conversion_buffer): Function deleted.
(struct conversion_buffer): New structure.
(MAX_ALLOCA): New macro.
(allocate_conversion_buffer): New macro.
(extend_conversion_buffer, free_conversion_buffer): New functions.
(ccl_coding_driver): Set coding->result.
(decode_coding): Set coding->result to CODING_FINISH_NORMAL if
this is the last block of source.
(encode_coding): Likewise.  Handle the source block as the last
one only when the whole source text is consumed.
(decode_coding_string): Handle the case that the output buffer is
too small to decode the whole source text.  Use
allocate_conversion_buffer, extend_conversion_buffer and
free_conversion_buffer, not get_conversion_buffer.
(encode_coding_string): Likewise.
(init_coding): Function deleted.
(init_coding_once): Delete code to initialize
conversion_buffer_size.