Split dns_name_copy() into dns_name_copy() and dns_name_copynf()

Split dns_name_copy() into dns_name_copy() and dns_name_copynf()

The dns_name_copy() function followed two different semanitcs that was driven
whether the last argument was or wasn't NULL.  This commit splits the function
in two where now third argument to dns_name_copy() can't be NULL and
dns_name_copynf() doesn't have third argument.

(cherry picked from commit f7aef3738a9288d6b85bdc9ef4496ca1120acac7)