mod_{dialback/s2s}, s2smanager: improve handling of s2s streams, (Fixes #78)
- If a stream to a remote entity which does multiplexing is opened, reference
the incoming multiplexed stream's session object on the outgoing one
- In the case there's a TLS failure, whenever opening an outgoing stream to a
remote entity which mandates TLS negotiation before dialback close the incoming
stream as well as long as that isn't multiplexed
mod_dialback, s2smanager: adjust code layout.