(enum define_coding_utf8_arg_index): New enum.

(enum define_coding_utf8_arg_index): New enum.
(enum coding_attr_index): Change coding_attr_utf_16_bom to
coding_attr_utf_bom.
(enum utf_bom_type): Renamed from utf_16_bom_type.
(struct utf_16_spec): Adjusted for the above change.
(struct coding_system): Add utf_8_bom in `spec' union.