Additional comments, checks, warning messages, and diagnostic
information is printed out when the client starts.

We assume IPv4 is always present and enabled. We assume IPv6 is not
supported when it is disabled at the command line or when it is not
supported by the firewall method. Warn if IPv6 is disabled but the
user specified IPv6 subnets, IPv6 DNS servers, or IPv6 excludes that
are effectively ignored.

Instead of indicating which features are on/off, we also indicate if
features are available in the verbose output.

We also more clearly print the subnets that we forward, excludes, and
any redirected DNS servers to the terminal output.

These changes should help handling bug reports and make it clearer to
users what is happening. It should also make it more graceful when a
user specifies a subnet/exclude with hostname that resolves to both
IPv4 and IPv6 (but IPv6 is disabled in sshuttle).