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(detect_coding_mask): Skip ASCII codes at the head according to
ascii_skip_code.
(coding_priorities): New variable.
(Fset_coding_priority_internal): New function.
(detect_coding): Call set_coding_mask with coding_priorities as
arg CODING.
(init_coding_once): Initialize ascii_skip_code.
(syms_of_coding): Declare set-coding-priority-internal as a Lisp
function.