Delete declaration for buffer-file-coding-system. It

Delete declaration for buffer-file-coding-system.  It
is done in buffer.c now.  In the comment, change coding-system to
coding system.  The name coding-vector is changed to coding-spec.
(coding-vector-type, coding-vector-mnemonic,
coding-vector-docstring, coding-vector-flags): Deleted.
(coding-system-spec-ref): New function.
(coding-system-type, coding-system-mnemonic, coding-system-flags):
Use coding-system-spec-ref.
(coding-system-doc-string): Renamed from coding-system-docstring.
(coding-system-eol-type): Renamed from coding-system-eoltype.
(coding-system-eol-type-mnemonic): Moved to mule-util.el.
(coding-system-post-read-conversion): Likewise.
(coding-system-pre-write-conversion): Likewise.
(default-process-coding-system): Deleted.  Now declared in
buffer.c.
(make-subsidiary-coding-system): New function.
(make-coding-system): Check arguments more strictly.  Do not make
-unix, -dos, -mac variants for TYPE 4.
(define-coding-system-alias): Call make-subsidiary-coding-system.
(set-buffer-file-coding-system): Adjusted for the function name
changes.
(find-new-buffer-file-coding-system): Likewise.
(default-process-coding-system): Deleted.  Now defined in coding.c.