MFC r359271:
Use IP_FW_NAT44_DESTROY opcode for IP_FW3 socket option to destroy NAT instance. The NAT44 group of opcodes for IP_FW3 socket option is modern way to control NAT instances and this method can be used in future to switch from numeric to named NAT instances, like was done for ipfw tables. The IP_FW_NAT_DEL opcode is the last remnant of old ipfw_ctl control plane that doesn't support versioned operations. This interface will be retired soon.