Include charset.h and category.h.

Include charset.h and category.h.
(Vsyntax_code_object): New variable.
(Fmodify_syntax_entry): Handle multibyte characters.  A multibyte
character in matching parenthesis is also handled correctly.  Use
shared object in the vector Vsyntax_code_object for an ASCII
character.
(describe_syntax): Handle a multibyte character in matching
parenthesis.
(describe_syntax_1): Describe also parent syntax tables.
(scan_words, Fforward_comment): Handle multibyte characters.
(scan_lists, char_quoted, Fbackward_prefix_chars): Likewise.
(scan_sexps_forward): Likewise.
(init_syntax_once): Initialize Vsyntax_code_object.
Initialize Vstandard_syntax_table by share objects in
Vsyntax_code_object.
(syms_of_syntax): Staticpro Vsyntax_code_object.