Added telnet option ECHO, to handle request #540

To turn off client local echo: ```self.caller.msg("", switchecho="off")```
To turn on client local echo: ```self.caller.msg("", switchecho="on")```

So, for example:
```
self.caller.msg("Please provide your password:", switchecho="off")
[...]
self.caller.sg("Password is correct. Welcome back", switchecho="on")
self.caller.sg("Now that is on, no need to do anything about it")
```