FIFO signaling producer that consumer has drained all bytes in pipe

[Florian Weimer]

* Jason Mehrens:

> I'm proposing that FileChannel should do what is done here:

> http://hg.openjdk.java.net/jdk/jdk/file/1871c5d07caf/src/java.base/unix/native/libjava/io_util_md.c#l193

It would need some research if FIONREAD works this way for all
supported targets.  I don't see any documentation that clearly tells
it's supposed to work on FIFOs.