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/param.rs

@@ -60,6 +60,9 @@ pub enum Param {
     /// For Messages
     WantsMdn = b'r',
 
+    /// For Messages: a message with Auto-Submitted header ("bot").
+    Bot = b'b',
+
     /// For Messages: unset or 0=not forwarded,
     /// 1=forwarded from unknown msg_id, >9 forwarded from msg_id
     Forwarded = b'a',