When decoding, prefer ptrdiff_t to int for buffer positions etc.

When decoding, prefer ptrdiff_t to int for buffer positions etc.

* coding.c (detect_coding_utf_8, emacs_mule_char)
(detect_coding_iso_2022, encode_coding_iso_2022, check_ascii)
(check_utf_8, decode_coding):
* coding.h (struct coding_system.errors):
Use ptrdiff_t, not int, for integer values derived from buffer and
string positions.