Revert "Invert the return value of spuc_add()."

This reverts commit ff7484d2cab0a27df97891da81cbc48955f1f973.

Instead of changing spuc_add() for consistency with spuc_remove(), this
commit changes spcu_remove() for consistency with the previous
spuc_add(), spuc_remove_all() and spuc_remove_all_types().

Returning true on an error is also consistent with errno and other error
handling software.