Only do the IPV6_V6ONLY setsockopt(3) stuff if that option is available.

Should fix #177, in which compilation failed on older Solaris systems
that didn't have it.  This is a different approach than a patch
suggested in that issue.

Weakly regression-tested on other platforms (test this by specifying
-6, -4, or neither to the server when binding to the wildcard address,
and seeing if a client can connect with various of -6, -4, or neither).