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.

lib/tests/test-mu-store-query.cc

@@ -1043,7 +1043,7 @@ https://trac.xapian.org/ticket/719
 
 	MemDb mdb;
 	Config conf{mdb};
-	conf.set<Config::Id::SupportNgrams>(true);
+	conf.set<Config::Id::NgramsEnabled>(true);
 
 	TempDir tdir;
 	auto store{make_test_store(tdir.path(), test_msgs, conf)};