send messages via SMTP in configurable chunks

providers may have a maximum for the SMTP RCPT TO: header,
therefore, an outgoing message is split into several chunks.
the chunk size defaults to 50, however, if known to be larger or smaller
by a dedicated provider, this setting may be changed.

this does not affect the MIME To: headers,
if a provider has a maximum here, this is change would not help.

deltachat-ffi/deltachat.h

@@ -353,6 +353,10 @@ char*           dc_get_blobdir               (const dc_context_t* context);
  * - `fetch_existing_msgs` = 1=fetch most recent existing messages on configure (default),
  *                    0=do not fetch existing messages on configure.
  *                    In both cases, existing recipients are added to the contact database.
+ * - `max_smtp_rcpt_to` = set the max. number of recipients that should be used for `RCPT TO:` smtp header.
+ *                    If a message needs to be sent to more recipients, multiple messages are sent out,
+ *                    each with exactly the same MIME-message (with `To:` mime headers that may contain all recipients)
+ *                    (defaults to 50)
  *
  * If you want to retrieve a value, use dc_get_config().
  *