ip/udp: Make input source address and source port saving MPSAFE

Change the global variable udp_in, udp_in6 and udp_ip6 into stack variable
and passing them around.  This should unbreak applications which depend
on received datagrams' source address or source port.
ifq: Expand IF queue operation macros

This prepares for the byte based queue limit and "rough priority queues"
netgraph7/source: ifnet.if_snd is not ifqueue
802.11: vap's if_snd is _not_ ALTQ ready
netgraph7/iface: Second parameter to ifq_dequeue should be NULL