(Qsignature, Qendian): Delete these variables.

(Qsignature, Qendian): Delete these variables.
(syms_of_coding): Don't initialize them.
(CATEGORY_MASK_UTF_16_AUTO): New macro.
(detect_coding_utf_16): Add CATEGORY_MASK_UTF_16_AUTO in
detect_info->found.
(decode_coding_utf_16): Don't detect BOM here.
(encode_coding_utf_16): Produce BOM if CODING_UTF_16_BOM (coding)
is NOT utf_16_without_bom.
(setup_coding_system): For a coding system of type utf-16, check
if the attribute :endian is Qbig or not (not nil or not), and set
CODING_REQUIRE_DETECTION_MASK if BOM detection is required.
(detect_coding): If coding type is utf-16 and BOM detection is
required, detect it.
(Fdefine_coding_system_internal): For a coding system of type
utf-16, check if the attribute :endian is Qbig or not (not nil or
not).