the attached patch keeps user charset preference in the
browser cookie.  http data is encoded according to that
preference.

charset may be toggled from page template like this:

 <td class="page-header-left"
  tal:define="uri
string:${request/base}${request/env/PATH_INFO}"
 >
  <a tal:attributes="href
python:request.indexargs_href(uri,
   {'@charset':'utf-8'})">utf-8</a>
  <a tal:attributes="href
python:request.indexargs_href(uri,
   {'@charset':'koi8-r'})">koi8-r</a>
 </td>