server: split off new server command 'queries'

Instead of being part of the ping command, create a separate
queries command, which is easier for updating. Ie. 'ping' implies "show
main screen" but we don't always want that side-effect.

lib/utils/mu-command-handler.cc

@@ -106,8 +106,7 @@ CommandHandler::invoke(const Command& cmd, bool do_validate) const
 	const auto cmit{cmap_.find(cmd.name())};
 	if (cmit == cmap_.cend())
 		return Err(Error::Code::Command,
-			   "unknown command in command '%s'",
-			   cmd.to_string().c_str());
+			   "unknown command '%s'", cmd.to_string().c_str());
 
 	const auto& cmd_info{cmit->second};
 	if (do_validate) {