My changes in r7738 (O_NONBLOCK for Unix Plink) were half-arsed, and

My changes in r7738 (O_NONBLOCK for Unix Plink) were half-arsed, and
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]