ToS low delay & acks

[Michal Soltys]

I've noticed that many programs love to "abuse" TOS byte, lowdelay bit in 
particular. The effect is, that a lot of packets that one would never 
expect, land in the 2nd queue (even scp & sftp set it), destroying in 
practice the purpose of having one.

I adjusted the condition in sys/net/pf.c in pf_test function, so only zero 
payload tcp acks are considered (it's working quite well so far).

Also I noticed, that the assignment to the 2nd queue, in case of ipv6, is 
checked only against lowdelay bit, and tcp ack is not considered at all. Why 
was it decided this way ?