Allow override options for the `Sender` header

[rodrigoaguilera]

From #51 (comment)

After reading the differences between From and Sender
https://www.xeams.com/difference-between-from-and-sender-header.htm

I think it can be useful for msmtp to allow rewritting this header. In fact, if I'm interpreting it right if Sender is present that should be the header that gets rewritten while preserving the original From header.

What do you think

[marlam]

Hm. Yes, according to the RFC, a sender-rewriting option such as set_sender_header (auto|on|off) would make sense.

On the other hand, the Sender header seems to be very seldomly used; a quick grep in my age-old mail archive shows only a bunch of very old and/or irrelevant examples (where From==Sender).

The usage example in the RFC is already something that never happens in real life: "For example, if a secretary were to send a message for another person, the mailbox of the secretary would appear in the "Sender:" field and the mailbox of the actual author would appear in the "From:" field." -- yeah right.

I'm not sure if this is worth it.