Convert junk filtering EPlugins to EExtensions.
We now have a proper junk mail filtering API. All junk filtering extensions must subclass EMailJunkFilter for user preferences and availability testing, and implement the CamelJunkFilter interface for the actual junk filtering and learning operations. The bogofilter module should be feature-equivalent to its former EPlugin. The spamassassin module is far more complex. It's nearly feature-equivalent to its former EPlugin, but I ditched the spamd respawning code since it seemed unnecessary for a mail client to have to deal with. If there's a huge outcry from users about it I'll reluctantly put it back, but I don't expect one. This gets us a step closer to killing off EConfig, and eventually the EPlugin framework itself.