NAT64 CLAT passes mbuf with IPv4 packet to nat64_do_handle_ip4(),
which is also shared with NAT64LSN and NAT64STL. nat64_do_handle_ip4()
expects that mbuf has full checksum, which is not always true.
For outbound traffic originating from the host running CLAT to the Internet,
TCP and UDP headers in mbuf contain pseudo checksum.
New parameter "local" hints nat64_do_handle_ip4() that mbuf has pseudo csum,
so that the full checksum can be calculated and passed to the Internet.
This parameter unbreaks the case when IPv6-only jail tries to reach IPv4-only
Internet, such as when jailed recursive resolver tries to resolve a domain
with IPv4-only glue records.
Submitted by: Evgeniy Khramtsov <evgeniy@khramtsov.org>