The 
[[external special remote protocol|design/external_special_remote_protocol]]
had extensibility built into it for messages git-annex sends, but not
for messages that the remote sends back to git-annex. To fix this
asymmetry, I've added a new EXTENSIONS to the protocol, which can be used
to find out about what new protocol extensions are supported.

There was the possibility that adding that might break some external
special remote that hardcoded the intial protocol messages. So, I checked
all of them that I know of, and all were ok, except for older versions of
datalad, which we were able to deal with. If you have your own external
special remote implementation, now would be a good time to check it.
