Eliminate Undefined variable errors in wpsc_obtain_the_description

This poorly-written function is now less poorly-written. Removed
unneeded globals, test $_GET variables before using, removed direct
query to database
Including Registration.php not required

Including registration.php not required  in versions since 3.1
Generates errors
Merge branch 'undefined_cat_id' into dominique
Merge branch 'wpsc_add_new_user-drop-registration-include' into dominique
Typo in last commit

Got too carried away
Merge branch 'undefined_cat_id' into dominique