smiley escaping fixes

smiley escaping fixes

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`.