Fix defaults.server.locked option to be working.

Currently defaults.server.locked option is not working. I can see the server name when i set the option, but can't connect to server.

In lib/server.js, preset_server is filtered by whether config.defaults.locked is on or off. By documentation, (and by template codes like assets/templates/connect.hbs), the config option to lock default server is "config.defaults.server.locked", not "config.defaults.locked". This patch fixes it.