Include charset.h.

Include charset.h.
(Fget_buffer_create): Allocate an extra byte for a buffer, and
make it always 0 for anchoring.
(reset_buffer): Handle a new member `enable_multibyte_characters'
in the struct buffer.
(fix_overlay_before): New function.
(init_buffer_once): Handle new members in the struct buffer.
(syms_of_buffer): Declare new buffer local variables
`enable-multibyte-characters' and `direction-reserved'.