Name generation plugins

[stuartpb]

Name generation plugins will provide a random-name hook, which will take a list of invalid names (which may have duplicates) and output a random name that isn't in that invalid name list on a single line. In the event that this hook produces multiple lines (say, two name generation plugins are installed), one will be picked at random.

The list of invalid names will itself come from a hook: for apps, this will be "invalid-app-names", and for services it will be "invalid-service-names" (or instance names or whatever). This will let plugins like "app-name-domain" include the results from an "subdomains-in-use" hook (since app names will conflict with other subdomains), which will pull any other subdomains added by other plugins (like web-exposed services). (It will also let you install blacklists which automatically disallow names you want to keep privileged for whatever reason.)