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`.
(cherry picked from commit
df3a8b183c4e6b50b05ba3c4514563619927c924)