remote: Forbid default "/session" connections when using ssh transport

remote: Forbid default "/session" connections when using ssh transport

Without the socket path explicitly specified, the remote driver tried to
connect to the "/system" instance socket even if "/session" was
specified in the uri. With this patch this configuration now produces an
error.

It is still possible to initiate a session connection with specifying
the path to the socket manually and also manually starting the session
daemon. This was also possible prior to this patch,

This is a minimal fix. We may decide to support remote session
connections using ssh but this will require changes to the remote driver
code so this fix shouldn't cause regressions in the case we decide to do
that.