new spam.el parameters: spam-autodetect and spam-autodetect-methods

[Ted Zlatanov <tzz@lifelogs.com>]

After testing it for a week, I added the spam-autodetect and
spam-autodetect-methods parameters (they have corresponding variables
you can customize if you don't want to use group/topic parameters).

spam-autodetect associates nil or t with groups or topics.  You could
use a regex to get all nntp groups, for instance.  When enabled,
spam-autodetect tells spam.el (through spam-find-spam, which is run
when you enter a buffer) that all UNSEEN articles should be checked
for spam.  This is done through the spam-autodetect-methods, which
can be a list of methods, or the default methods you use for
spam-split anyway.  spam-autodetect-methods is also associated with
groups or topics in a flexible way - as a parameter or a regex.

There are two things: the registry can be optionally used to make sure
that messages (by ID) that have already been tagged as spam don't get
rechecked, and the user can ask that ALL, not just the unseen
articles, be checked when you enter the buffer.

Spam autodetection is a little slow, because spam-split can currently
only be invoked on one article at a time.  I'll see if I can improve
this, but meanwhile keep in mind that fast checks are best.

Finally, the new parameters have made group/topic parameters (`G c')
very slow to generate for me.  Maybe it something in my setup, but it
might be a bug in my customization declarations.  Please let me know
if it's the latter.

This is the last change to spam.el I am making for No Gnus.  From now
on, only bug fixes and documentation updates will be made.  I'll
update the Gnus manual to take note of all the spam.el changes.
Thanks for everyone's patience with me.

Ted