Checkmk Werk 12296: Fix IPv6 handling in dual stack

ID: 12296 Title: Fix IPv6 handling in dual stack Component: Checks & agents Level: 1 Class: Bug fix Version: 2.1.0i1 Selecting IPv6 as the default of a dual stack configuration would break the communication with the host with the following error: "Communication failed: [Errno -9] Address family for hostname not supported".