6884800: Path.newInputStream does not usefully implement available()

[Alan Bateman]

I need a reviewer for this small change.  A long standing issue 
(4648049) is that the InputStream returned by 
Channels.newInputStream(ReadableByteChannel) doesn't usefully implement 
the available method. 6884800 is the case where the channel is to the 
file, a case where available can be easily implemented. The webrev is here:
  http://cr.openjdk.java.net/~alanb/6884800/webrev.00/

Thanks,
Alan.