(charset_ordered_list_tick): New variable.

(charset_ordered_list_tick): New variable.
(Fdefine_charset_internal): Increment charset_ordered_list_tick.
(Funify_charset): New optional arg DEUNIFY.  If it is non-nil,
deunify intead of unify a charset.
(string_xstring_p): Add `const' to local variables.
(find_charsets_in_text): Add `const' to arguemnts and local
variables.
(encode_char): Adjusted for the change of Funify_charset.  Fix
detecting of invalid code.
(Fset_charset_priority): Increment charset_ordered_list_tick.