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.