R1:9de54aedb578

Remove inconsistent and confusing use of the term "multiplex" in mail

Summary:
Ref T13053. Because I previously misunderstood what "multiplex" means, I used it in various contradictory and inconsistent ways.

We can send mail in two ways: either one mail to everyone with a big "To" and a big "Cc" (not default; better for mailing lists) or one mail to each recipient with just them in "To" (default; better for almost everything else).

"Multiplexing" is combining multiple signals over a single channel, so…