[BUG] Custom headers in `notmuch-message-headers` are broken

[Jan Malakhovski]

> 3) We should think carefully about whether we want to blindly send
>    certain large headers like "Received". Some people use notmuch via
>    ssh or equivalent, and it might (dunno) be a concern.

I'd prefer `notmuch show` to dump everything by default and have an
option like `--headers` to limit those. I.e. to get current behavior
you'd just dump comma-separated `notmuch-message-headers` into that
option in `notmuch.el` and be happy.

> 1) You'll want to avoid duplicating headers already emitted

Why? Wouldn't that prevent you from parsing "Received" headers in
`notmuch.el`?