add a flag field to xtags so that we can filter tags based on whether or not the parent xchan is safe or not. Otherwise we'll have tags that lead to nowhere because the directory entry is hidden but the tag isn't. A successful porn site in the matrix could also swamp the directory with x-rated tags, even if the site was playing nice and did everything right to self-censor. Accomplishing this with joins would be horrendously inefficient, though it will take a bit of code re-org to get this flag where it needs to be when it's time to set keywords.