replace the use of struct ifqueue in pipex with mbuf_queues.

replace the use of struct ifqueue in pipex with mbuf_queues.

this has a slight semantic change. previously pipex would only
process up to 128 packets on the input and output queues at a time
and would reschedule the softint if there were any left. now it
mq_delists the current set of pending packets and only processes
them. if anything is added to the queues later they'll cause the
softint to run again.

this in turn lets us deprecate sysctl_ifq since nothing uses it
anymore. because niqueues are mostly wrappers around mbuf_queues,
we can provide sysctl_mq and just #define sysctl_niq to it.

pipex bits are ok yasuoka@