[MlMt] don't like auto-add

[Randy Bush]

> The standard mailboxes are part of each source and are required for
> proper operation as far as I know.
> 
>> my issue is that i do not want to be gratuitously subscribed to new
>> mailboxes in SOURCES.  i maintain the list very carefully, and have
>> a length constraint due to screen real estate.

am i that unclear?  i do not care if mm makes 1,000 magic folders.  just
don't force me to see them, damn it.

randy