properties: support system-properties

Support "system" properties, i.e., library versions, whether
language-detection, scm is supported.

Make these more generic (with display strings), so we can handle them
more automatic int mu-cmd-info.

mu/mu-cmd-init.cc

@@ -61,7 +61,7 @@ Mu::mu_cmd_init(const Options& opts)
 		if (!opts.init.ignored_addresses.empty())
 			conf.set<Config::Id::IgnoredAddresses>(opts.init.ignored_addresses);
 		if (opts.init.support_ngrams)
-			conf.set<Config::Id::SupportNgrams>(true);
+			conf.set<Config::Id::NgramsEnabled>(true);
 
 		return Store::make_new(opts.runtime_path(RuntimePath::XapianDb),
 				       opts.init.maildir, conf);