(Qutf16): New variable in MULE.

(Qutf16): New variable in MULE.
(Fmake_coding_system): Accept `utf-16' as `type' in MULE.
(Fcoding_system_type): Add `utf-16' in MULE.
(struct detection_state): Add a structure for utf16 in MULE.
(detect_coding_type): Setup st->utf16.mask in MULE.
(mule_decode): Use `decode_coding_utf16' for `CODESYS_UTF16' in MULE.
(reset_encoding_stream): Use `char_encode_utf16' and
`char_finish_utf16' for `CODESYS_UTF16' in MULE.
(detect_coding_utf16): New function [incomplete].
(decode_coding_utf16): New function [support only UCS-2].
(char_encode_utf16): New function.
(char_finish_utf16): New function.
(syms_of_file_coding): Add new symbol `utf-16'; setup
`coding_category_symbol[CODING_CATEGORY_UTF16]'.