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.