spam.el is customizable now

[Ted Zlatanov <tzz@lifelogs.com>]

I did this in preparation for adding the group parameters, but it
turned out to be fun in itself...

All the defvars are defcustom now, so you can just customize the
"spam" group.  There is a spam-bogofilter group under it.  I renamed a
few minor variables, which probably won't affect anyone, but go
through the customization if you use spam.el to make sure you get the
settings right.

The only problem I had is in making a customizable list of marks to be
considered spam/ham marks.  Right now it's a list of characters, but
it would be better if it was a list of mark choices, with each choice
inserting the correct character in the list.  The list of marks can be
static, but it would be perfect if it came from the authoritative Gnus
list of marks.  If anyone is interested in doing this, feel free.

Thanks
Ted