some progress on the smtp bounce proxy

some progress on the smtp bounce proxy

we don't want to make distinction between permanent or temporary
errors, because we should add a "unknown" level, and that will be too
complicated for really to little gain.

so we have basicaly :
 - ignore (drop the mails)
 - notice (forward the "bounce" as is - default)
 - error  (do the broken leg + forge a new bounce)

the errors levels are regexp-driven (read from a mysql db)

the only thing left is the treatment of the ERROR level (others are
fine, and the web interface to edit the regexps too)

git-archimport-id: opensource@polytechnique.org--2005/platal--mainline--0.9--patch-117