Log Message:
When building without any mdns support on NetBSD (at least), configure
will still find the base mdnssd, and build and install ippfind
support. This will lead to a PLIST error -- as far as pkgsrc is
concerned, we did not enable it.

Tell configure explicitly to not add mdns support when we don't want any.

While here, group mdnssd and avahi as optional and mutually exclusive.