use constants in get_pref()/set_pref()

classes/api.php

classes/article.php

classes/digest.php

classes/feeds.php

classes/handler/public.php

classes/pref/feeds.php

classes/pref/filters.php

classes/pref/prefs.php

classes/rpc.php

classes/rssutils.php

classes/sanitizer.php

classes/timehelper.php

classes/userhelper.php

include/controls_compat.php

include/functions.php

index.php

plugins/close_button/init.php

prefs.php