completely broke interactive logins. The problem, or at least one of the
problems, was that in interactive use stdin, stdout, and stderr tend to be
the same file, so setting O_NONBLOCK on the latter two also sets it on the
former. Thus, we need to cope with all of them being non-blocking.
[originally from svn r7742]
[r7738 == d0db31a1ca9675ae0db3dd5123236a60f9a3e2cb]