Avoid generating duplicate headers in ezmlm-send

When processing a message's headers:

 - Make note if Reply-To was seen.
 - Save off and do not copy the Cc header to the output immediately.
 - Save off and do not copy the From header to the output immediately.

At the end of the message headers, call rewrite_from().

When flagrewritefrom is set, either by the list's configuration or
because a DMARC reject policy applies to the message sender's address:

 - The From header is rewritten as before.
 - If replytolist is not enabled, a Reply-To header is only added
   if the original message did not have one. Rationale is that the
   original Reply-To header indicated where the post author is
   interested in receiving responses, not the From header.
 - When replytolist is enabled and the original message had a Cc header,
   the From address is appended to the existing Cc header.
 - When replytolist is enabled and the original message did not have a
   Cc header, generate a new one using the From address.

After rewrite_from() performs any of the applicable manipulations
described above, the resulting From and Cc headers are copied to the
output.

Update tests/551-ezmlm-send-rewritefrom:

 - Confirm Reply-To and Cc headers are not duplicated.
 - Validate behavior when replytolist is configured.