PATCH: silent subscription

Got bitten by this one, too! The documentation for mlmmj-sub has an interesting
use of the word 'silent'. I've changed the documentation and added an option to
actually allow silent subscription even when moderation is turned on.

I also added a tunable to disable the message to the requester. Mail moderation
doesn't spam the requester each time they send a mail, so why should subscription
requests? (Though notifying posters that their mail is awaiting moderation is a
feature I will need, too, and will send a patch shortly for that as it should be
fairly straightforward.)

Patch attached. Could it be included in mlmmj?

Cheers,

Ben.