Announce new prefdir, preferences functions and add to manual. * NEWS: Add prefdir, preferences to list of new functions for 3.8. * doc/interpreter/gui.txi: Add prefdir, preferences to manual. * scripts/help/__unimplemented__.m: Remove functions from list of unimplemented. * scripts/prefs/prefdir.m: Add @seealso references to docstring. * scripts/prefs/preferences.m: Use prefsfile() instead of creating name. * scripts/prefs/private/prefsfile.m: Use filesep() instead of '/' for Windows users.