Opening an empty FIFO blocks

The following hangs until something is written to `test`:
```julia
fifopath = Libc.mkfifo("test")
iofifo = open(fifopath, "r")
```

I think it should be possible to open a fifo regardless of whether there is data yet or not. If there is no data, probably `eof(iofifo)` should be true immediately after `open`ing.