Documentation,selinux: fix references to old selinuxfs mount point

selinuxfs was originally mounted on /selinux, and various docs and
kconfig help texts referred to nodes under it.  In Linux 3.0,
/sys/fs/selinux was introduced as the preferred mount point for selinuxfs.
Fix all the old references to /selinux/ to /sys/fs/selinux/.
While we are there, update the description of the selinux boot parameter
to reflect the fact that the default value is always 1 since
commit be6ec88f ("selinux: Remove SECURITY_SELINUX_BOOTPARAM_VALUE")
and drop discussion of runtime disable since it is deprecated.

Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>