nixos/darkhttpd: change listening address default to ::1

We unconditionally pass --ipv6 to the binary. In older versions, setting
both --addr 127.0.0.1 and --ipv6 resulted in listening on [::],
a potential security issue. On the other hand, the binary started.

With the current version however, --ipv6 implies an IPv6 address in
--addr resulting in a crash with: "darkhttpd: malformed --addr argument"

While this commit is technically a BC-break, it should be harmless as
existing users either do not explicitly set the address and the server
refuses the start (making this change rather a fix) or they set the
address and are therefore unaffected.

nixos/modules/services/web-servers/darkhttpd.nix

@@ -27,7 +27,7 @@ in {
     };
 
     address = mkOption {
-      default = "127.0.0.1";
+      default = "::1";
       type = str;
       description = ''
         Address to listen on.