Permit truncating table product

Permit truncating table product

Truncating is actually fine for table product.  It's forbidden because
selector sname has flag NSC_CONST set.  I don't remember why sname was
made const in commit 445dfec9 along with item selector mnem, sect-chr
selector mnem and infrastructure selector name.  Unlike the other
tables, no code depends on product's builtin values.  Clear the flag.