Add support for TCP_KEEPIDLE/ TCP_KEEPINTVL socket options (lnx)

The Oracle JDBC team are interesting in having support for these socket options.

At this time, these socket options are mostly on Linux. We should at least explore the issues, maybe the right thing is to make they available via a JDK-specific API, maybe the right thing is to define a JDK-specific SocketOption so that it can be used via SocketChannel's set/getOption methods.