Reduce <sys/queue.h> pollution.

While <sys/sysctl.h> includes <sys/queue.h> unconditionally, it is only
actually used in code which is conditional on _KERNEL. Make the #include
itself conditional as well, and fix userland code that uses <sys/queue.h>
for other purposes but relied on <sys/sysctl.h> to bring it in.

MFC after: 1 week