[Standards] dialback: piggybacking

[Philipp Hancke]

Peter Saint-Andre wrote:
>> Reads okay ... but as you said it should be specified how error flows 
>> are handled. I.e. how a server not supporting piggybacking does refuse 
>> its use and how a server trying to do piggybacking has to react when 
>> the other server refuses its use.
> 
> Yes, let's fix that up.

What about signalling support in the stream feature?
Like
<stream:features>
   <dialback xmlns='urn:xmpp:features:dialback'>
     <required/>
     <piggybacking/>
   </dialback>
</stream:features>

This might avoid some of the error cases.

Philipp