Fix carbons criteria

We came into the carbons checking code when we received `<private
xmlns="urn:xmpp:carbons:2"/>`. Which actually marks a message to _not_
be a carbon.

In this code we also make sure that carbons only come from us.
If not we don't call the message handler code.

So we should actually only check for `<sent xmlns='urn:xmpp:carbons:2'>`
and `<received xmlns='urn:xmpp:carbons:2'>`.

Thanks pukkamustard and Holger.

Fixes https://github.com/profanity-im/profanity/issues/1614

src/xmpp/stanza.h

@@ -117,6 +117,8 @@
 #define STANZA_NAME_USERNAME         "username"
 #define STANZA_NAME_PROPOSE          "propose"
 #define STANZA_NAME_REPORT           "report"
+#define STANZA_NAME_RECEIVED         "received"
+#define STANZA_NAME_SENT             "sent"
 
 // error conditions
 #define STANZA_NAME_BAD_REQUEST             "bad-request"