Use Auto-Submitted: auto-generated header to identify bots

New `dc_msg_is_bot()` C API and corresponding `Message.is_bot()`
Python API can be used to check if incoming message is sent by a bot,
e.g. to avoid two echo bots replying indefinitely to each other.

"Bot" flag is not set for outgoing messages, but may be set for
BCC-self messages. For now documentation says that `dc_msg_is_bot()`
return value is unspecified for outgoing messages. It can be better
specified later if needed for specific applications, e.g. sharing an
account with a helper bot.

src/mimeparser.rs

@@ -498,6 +498,12 @@ impl MimeMessage {
 
             self.parts.push(part);
         }
+
+        if self.header.contains_key("auto-submitted") {
+            for part in &mut self.parts {
+                part.param.set(Param::Bot, "1");
+            }
+        }
     }
 
     async fn avatar_action_from_header(