[8u] RFR(m): 8194298: Add support for per Socket configuration of TCP keepalive

[Severin Gehwolf]

Hi,

Please review this OpenJDK 8u backport of JDK-8194298 which adds socket
configuration options for TCP keepalive. It's one of the Oracle JDK
parity patches.

As with the OpenJDK 11u change, this includes Linux and Mac only
changes. This backport is pretty much a rewrite as the JDK 11u codebase
is rather different in this area.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8194298
8u CSR: https://bugs.openjdk.java.net/browse/JDK-8236187
webrev: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8194298/jdk8/04/webrev/

Testing:
 * Build/test on Linux x86_64 and Mac OS x (thanks to Simon Tooke).
   Build on Windows x86_64 (also thanks to Simon Tooke)
 * tier1 tests, test/java/nio test/java/net/ test/jdk/net/ on Linux
   x86_64. No regressions noted.

Thoughts?

Thanks,
Severin