Update /proc/spl/kmem/slab output

The 'slab_fail', 'slab_create', and 'slab_destroy' columns in the slab
output have been removed because they are virtually always zero and
not very useful.

The much more useful 'size' and 'alloc' columns have been added which
show the total slab size and how much of the total size has been
allocated to objects.

Finally, the formatting has been updated to be much more human
readable while still being friendly for tool like awk to parse.