* -CSD does not affect modules, so readfile() was not using the utf-8 input

* -CSD does not affect modules, so readfile() was not using the utf-8 input
  layer, which led to lots of problems; make it force read files as utf-8.
  Closes: #373203
* writefile() likewise needs to use the utf8 output layer.
* Remove the -CSD from ikiwiki's hashbang since it's useless to have it
  there.
* Revert some of the decode_utf8 changes in CGI.pm that seem unnecessary
  given the readfile fix.
* Add utf-8 testcases for readfile and htmlize.