data-control: Don't send offers to the clients that sent them

If a client is listening for offers and sends an offer, it will block forever when it attempts to receive the offer and read from the file descriptor. This is because the offer is sent back to the client that sent it. Since the writing and reading ends are in the same client, it never has a chance to write (barring methods to work around this problem). Even if the client could read its own selection, it is pointless to do so.

Here we fix the problem by not making offers back to the client that sent them. The protocol has been updated to reflect this change. Closes #2406 (closed).