rename dns_name_copynf() to dns_name_copy()

The API for copying names is a little more complicated than it needs to be. One thing we can improve is to eliminate one of the two separate copying functions, dns_name_copy() and dns_name_copynf(). copy is used in only two places, so we change it to copynf in those places, then remove the copy implementation and rename the copynf function to copy.