This is the first patch of a (long) series aimed at gradually
replacing xnqueue data structures with regular list_head
objects.

Legacy Xenomai queues and kernel lists will live in parallel, until
the rebase is complete, at which point include/cobalt/kernel/queue.h
will be dropped from the tree.

The motivation is to better comply with kernel standards and remove
all useless overhead and specifics about Xenomai queues/lists.

For instance, the element count maintained by the xnqueue object is
most often used only for testing for emptiness, which makes it
overkill. The few users of the actual item count should rather do
their own accounting instead.