Now, we only set O_NONBLOCK on file descriptors that we create
ourselves.  File descriptors that we inherit (stdin, stdout, stderr)
are kept in blocking mode.  The way we deal with this differs between
the threaded and non-threaded runtimes:

 - with -threaded, we just make a safe foreign call to read(), which
   may block, but this is ok.

 - without -threaded, we test the descriptor with select() before
   attempting any I/O.  This isn't completely safe - someone else
   might read the data between the select() and the read() - but it's
   a reasonable compromise and doesn't seem to measurably affect
   performance.