pf: teach nat64 to handle 0 UDP checksums

For IPv4 it's valid for a UDP checksum to be 0 (i.e. no checksum). This isn't
the case for IPv6, so if we translate a UDP packet from IPv4 to IPv6 we need to
ensure that the checksum is calculated.

Add a test case to verify this. Rework the server jail so it can listen for TCP
and UDP packets at the same time.

Sponsored by: Rubicon Communications, LLC ("Netgate")