From: Stephen Smalley <sds@epoch.ncsc.mil>

This patch adds a kernel configuration option that enables writing to a new
selinuxfs node 'disable' that allows SELinux to be disabled at runtime prior
to initial policy load.  SELinux will then remain disabled until next boot.
This option is similar to the selinux=0 boot parameter, but is to support
runtime disabling of SELinux, e.g.  from /sbin/init, for portability across
platforms where boot parameters are difficult to employ (based on feedback by
Jeremy Katz).