(Q_compatibility_of): New variable.

(put_char_composition): Use XCAR/XCDR instead of Fcar/Fcdr if an
object is proved to be a cons cell; return `<-compatibility' if the
first element of argument `value' is `compat'.
(Fput_char_attribute): Eliminate the first value if
`put_char_composition' returns `<-compatibility'.
(Fsave_char_attribute_table): Use `Fchar_refs_simplify_char_specs' for
`<-compatibility'.
(syms_of_chartab): Add new symbol `<-compatibility'.