Generalize INTERNAL_FIELD between buffers, keyboards and frames.

Generalize INTERNAL_FIELD between buffers, keyboards and frames.
* src/lisp.h (INTERNAL_FIELD): New macro.
* src/buffer.h (BUFFER_INTERNAL_FIELD): Removed.
(BVAR): Change to use INTERNAL_FIELD.
* src/keyboard.h (KBOARD_INTERNAL_FIELD): Likewise.
(KVAR): Change to use INTERNAL_FIELD.
* src/frame.h (FVAR): New macro.
(struct frame): Use INTERNAL_FIELD for all Lisp_Object fields.
* src/alloc.c, src/buffer.c, src/data.c, src/dispnew.c, src/dosfns.c
* src/eval.c, src/frame.c, src/fringe.c, src/gtkutil.c, src/minibuf.c
* src/nsfns.m, src/nsterm.m, src/print.c, src/term.c, src/w32fns.c
* src/w32menu.c, src/w32term.c, src/window.c, src/window.h, src/xdisp.c
* src/xfaces.c, src/xfns.c, src/xmenu.c, src/xterm.c: Users changed.
* admin/coccinelle/frame.cocci: Semantic patch to replace direct
access to Lisp_Object members of struct frame to FVAR.