See bug #411786. Perl's random corruption of the taint flag is even effecting
the untainting of source filenames now (which AFAICS, is a proper untaint
and always worked before..), and that makes using ikiwiki in perl taint
mode not work at all.
* Plugins can add new directories to the search path with the add_underlay
function.
* Split out smiley underlay files into a separate underlay, so if the plugin
isn't used, the wiki isn't bloated with all those files.