[MERGE][0.16] Fix incompatibility with <= 0.15 servers ("bad request 'Transport.is_readonly'")

[Robert Collins]

Robert Collins has voted +1 (conditional).
Status is now: Conditionally approved
Comment:
'pre 0.16 clients' might read better as 'clients before bzr 0.16'. Also, 
the new parameter to RemoteTransport should have a :param: item in the 
docstring.

+1 with that for bzr.dev, and I'll pull it from there to 0.16.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C20070430020640.GA16437%40steerpike.home.puzzling.org%3E