pf: make TCP sequence number tracking less strict by one octet for FIN packets

The data of a TCP packet must fit into the announced window, but this is not
required for the sequence number of the FIN. A packet with the FIN bit set and
containing data that fits exactly into the announced window was blocked. Our
stack generates such packets when the receive buffer size is set to 1024. Now
pf uses only the data lenght for window comparison.
OK henning@

Obtained From: OpenBSD
Sponsored by: Rubicon Communications, LLC ("Netgate")