On HPUX, Solaris, Tru64 (Dec UNIX), and IRIX (I think),
O_NONBLOCK is the POSIX version of non-blocking I/O
which is what we want.

On Linux and FreeBSD (at least), O_NONBLOCK and O_NDELAY are the same.
So this change should have no negative effect on those platforms.

Tested on Linux, Solaris, HPUX.

Thanks to Anders Qvist for diagnosing this problem.