RFR:8194298 Add support for per Socket configuration of TCP keepalive

[vyom tewari]

Hi All,

Please review the below code.
BugId    : https://bugs.openjdk.java.net/browse/JDK-8194298

webrev : http://cr.openjdk.java.net/~vtewari/8194298/webrev0.0/index.html

Currently Java supports SO_KEEPALIVE, whose default value is 7200 
seconds which is too long for most of the applications. This code change 
will allow us to set the keepalive 
parameters(TCP_KEEPIDLE,TCP_KEEPCNT,TCP_KEEPINTVL)  which will configure 
the idle time on per socket basis.

I did code changes for Linux & Mac only, support for other platforms can 
be added in future if needed.

Thanks,

Vyom