Add support for some getsockopt/setsockopt options.

I would like to add support for getting/setting SO_RCVTIMEO and SO_KEEPALIVE socket option values. The motivation for this change is to get the [docker-client](https://github.com/spotify/docker-client) library to switch from using unix-socket-factory (junix-socket) to jnr-unixsocket ([spotify/docker-client Issue 98](https://github.com/spotify/docker-client/issues/98)).

Currently, getsockopt/setsockopt only support boolean/integer values but an option like SO_RCVTIMEO needs its value to be a struct timeval (sys/time.h).

This change introduces :
- Support for getting/setting a 'struct timeval' in the Native class along with
dependence on jnr-posix
- Method in UnixSocketChannel to get/set SO_KEEPALIVE
- Method in UnixSocketChannel to get/set SO_RCVTIMEO