RFR [12] 8212000: Verify exported symbols in java.base (libnet, libnio/ch)

[Pavel Rappo]

Hello,

Please review the following change:

  http://cr.openjdk.java.net/~prappo/8212000/webrev.00/8212000/

This is a clean-up kind of change. It seals unnecessarily exported functions
from libnet and libnio, and also removes an unused declaration from libnet. 
This change has been built and tested on all supported platforms.

Thanks,
-Pavel