This marks all non-network PPP traffic with the L2TP priority bit. The
packet queue is also split (albeit inefficiently) into control/priority
and non-priority without re-ordering within each type.

Identify priority traffic with:
	-d PEER -p udp --dport 1701 ! -f -m u32 ! --u32 0>>22&0x3C@8>>24&0x81=0
and normal traffic with:
	-d PEER -p udp -f
	-d PEER -p udp --dport 1701 ! -f -m u32 --u32 0>>22&0x3C@8>>24&0x81=0

Traffic can then have appropriate TOS and TC class applied, to avoid
discarding vital traffic like L2TP HELLO and LCP EchoReq/EchoRep.