sshreadproc() needs to be started after opening the sshfd file
descriptor.
fsnetproc() needs to run in the same filedescriptor group as
the fileserver.
return the error message from MSG_CHANNEL_OPEN_FAILURE
in the "connect" control write.
use a extra state "Finished" to distinguish server from client
initiated teardown. that way we do not need to track if we
send the MSG_CHANNEL_CLOSE message in closeclient(). this way
we also cannot be fooled by misbehaving server.
simplify hangupclient() by removing state transitions and doing
them in the caller explicitely. that way we can use hangupclient()
instead of dialedclient().
