introduce mbuf list and queue apis. both manage fifo lists of mbufs

introduce mbuf list and queue apis. both manage fifo lists of mbufs
and a count of the mbufs.

struct mbuf_list and the ml_foo() apis can be used to build lists of
mbufs where you dont need locking (eg, on the stack).

struct mbuf_queue and mq_foo() wrap mbuf_lists with a mutex, and
limits the number of mbufs that can be queued. they can be useful
for moving mbufs between contexts/subsystems.

with help from jmc@ for the manpage bits
mpi@ is keen