RFR: 7131399: Poll system call appears to be broken on Mac OS [macosx]

[Michael McMahon]

Can I get the following change reviewed please?

http://cr.openjdk.java.net/~michaelm/7131399/webrev.1/
The problem is that poll(2) doesn't seem to work in a specific edge case 
tested by JCK,
namely, when a zero length UDP message is sent on a DatagramSocket.  The 
problem is only
detected on timed reads, ie. normal blocking reads work fine.

The fix is to make the NET_Timeout() function use select() instead of 
poll().

Thanks,
Michael.