Discovered with the test suite, and reproduced directly.

Trying to connect to hub.darcs.net with darcs built against
`tls-2.0.0`, we get

```
Error getting "https://hub.darcs.net/darcs/darcs-screened/_darcs/format": InternalException (HandshakeFailed (Error_Protocol 
"peer does not support Extended Main Secret" HandshakeFailure))
And also:
Error getting "https://hub.darcs.net/darcs/darcs-screened/_darcs/inventory": InternalException (HandshakeFailed 
(Error_Protocol "peer does not support Extended Main Secret" HandshakeFailure))
```

The immediate trigger is that between tls-1.9.0 to tls-2.0.0,
the default setting for this was changed from "AllowEMS" to
"RequireEMS".

I've tried overriding this setting from darcs but ran into
other problems with overriding any TLS settings.

For now I'll probably constrain tls to <2.0.0. The best
long-term fix would probably be to update hub.darcs.net,
or alternatively figure out how to override the TLS settings.