If we're printing password prompts to /dev/tty rather than standard

If we're printing password prompts to /dev/tty rather than standard
error, we should also read the corresponding password inputs from
/dev/tty. That way, redirection of Plink's standard input will play
nicely with SSH sessions that need interactive login.

(This is what we get for disdaining getpass(3) and going it alone, of
course. But we had no choice, due to the extra output part way through
keyboard-interactive.)

[originally from svn r9262]