Smileys need to be double-escaped to work, since the smiley plugin runs as
a sanitize hook, and markdown helpfully removes one level of escapes first.
There were some bugs in the smiley handling code that made escaped smileys
still be expanded. After unescaping a smiley, it needed to move pos forward
past it or the next pass would expand it.
Also, once the m//g got to the end, it seemed to loop back through and make
one more pass (a difference in perl 5.10's regexp exngine? I observed that
pos was undefined when this happened, so added a `last unless defined pos`.
* 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.