Revert and cleanup Lisp_Cons, Lisp_Misc and Lisp_Symbol things.

Revert and cleanup Lisp_Cons, Lisp_Misc and Lisp_Symbol things.
* src/lisp.h (struct Lisp_Symbol): Change xname to meaningful
name since all xname users are fixed long time ago.  Do not use
INTERNAL_FIELD.
(set_symbol_name, set_symbol_function, set_symbol_plist):
(set_symbol_next, set_overlay_plist): New function.
(struct Lisp_Cons): Do not use INTERNAL_FIELD.
(struct Lisp_Overlay): Likewise.
(CVAR, MVAR, SVAR): Remove.
* src/alloc.c, src/buffer.c, src/buffer.h, src/bytecode.c:
* src/cmds.c, src/data.c, src/doc.c, src/eval.c, src/fns.c:
* src/keyboard.c, src/lread.c, src/nsselect.m, src/xterm.c:
Adjust users.
* src/.gdbinit: Change to use name field of struct Lisp_Symbol
where appropriate.
* admin/coccinelle/overlay.cocci, admin/coccinelle/symbol.cocci:
Remove.