Overhaul m-files in prefs directory. Update docstrings, re-code to match Matlab behavior, put input validation first, add BIST tests. * addpref.m, getpref.m, ispref.m, preferences.m, rmpref.m, setpref.m: Update docstrings, re-code to match Matlab behavior, put input validation first, add BIST tests. * loadprefs.m, prefsfile.m, saveprefs.m: Update docstring. Remove ## note about testing. * saveprefs.m: Update docstring. Remove ## note about testing. Change function declaration to not return any value. * scripts/prefs/private/prefdir.m: Move file from scripts/prefs to private directory. Match variable names in doc to those in function. * module.mk: Change build system to find prefdir.m in private directory.