(CODING_UTF_8_BOM): New macro.

(CODING_UTF_8_BOM): New macro.
(enum coding_category): Delete coding_category_utf_8, add
coding_category_utf_8_auto, coding_category_utf_8_nosig, and
coding_category_utf_8_sig.
(CATEGORY_MASK_UTF_8): Delete it.
(CATEGORY_MASK_UTF_8_AUTO, CATEGORY_MASK_UTF_8_NOSIG)
(CATEGORY_MASK_UTF_8_SIG): New macros.
(CATEGORY_MASK_ANY): Delete CATEGORY_MASK_UTF_8, add
CATEGORY_MASK_UTF_8_AUTO, CATEGORY_MASK_UTF_8_NOSIG, and
CATEGORY_MASK_UTF_8_SIG.
(CATEGORY_MASK_UTF_8): New macro.
(UTF_BOM, UTF_8_BOM_1, UTF_8_BOM_2, UTF_8_BOM_3): New macros.
(detect_coding_utf_8): Check BOM.
(decode_coding_utf_8, encode_coding_utf_8): Handle BOM.
(decode_coding_utf_16): Adjusted for the change of enum
utf_bom_type.
(encode_coding_utf_16): Likewise.
(setup_coding_system): Likewise.  Set CODING_UTF_8_BOM (coding).
(detect_coding, detect_coding_system): Handle utf-8-auto.
(Fdefine_coding_system_internal): Handle `bom' property for utf-8.
(syms_of_coding): Fix setting up of Vcoding_category_table.