(allocate_itemsets): Use xnmalloc, not xcalloc,

(allocate_itemsets): Use xnmalloc, not xcalloc,
to allocate kernel_base, kernel_items, kernel_size, since
they needn't be initialized to 0.
(set_states): Don't reuse kernel_size and kernel_base
for initial core.  Just have a separate core, so we needn't worry
about whether kernel_size and kernel_base are initialized.
(shift_symbol, redset, shiftset, kernel_base,
kernel_size, kernel_items): Remove unnecessary initialization.