Account for extra message overhead added by server when splitting

[wjt]

This doesn't take into account the "relay prefix" that the server will add to your messages before forwarding them to their recipients. See this logic in telepathy-idle for an attempt (hopefully successful, but who knows) to work out what the prefix will be. It depends on what the server believes your Unix username and hostname to be; so the client WHOISes itself to work these out.

[meshy]

Thank you for this -- I hope I get the time to look into this soon :)