how can i start the screensaver (and lock the screen) from the os x terminal?

Have you tried this?

/System/Library/Frameworks/ScreenSaver.framework/Resources/ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine

Update for macOS High Sierra:

/System/Library/CoreServices/ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine