Protecting /proc/slabinfo

chmod() works fine on `/proc`, so rather than tilting at windmills trying to change long-established kernel interfaces upstream, these changes should be suggested for addition to distro startup scripts. Permissions on pseduo-terminals have long been handled in this way.

That will immediately advantage even people running older kernels, and the distros are in the best place to shake out (and fix) any userspace breakage.