open() should support /dev/tty.

This is fixed but reverted the changes because password authentication testcases are failing.
With this fix, when we have a terminal SSH will always reads the password from the terminal. This makes our password authentication testcases to fail as there is no way to pass the password through SSH_ASKPASS.

As of now, /dev/tty is only used in the readpassphrase so for now we are not implementing the /dev/tty support.