Completely empty/unavailable $::selinux fact means no SELinux.

I have few RHEL machines with completely disabled SELinux and latest facter fails on $::selinux get:

# sestatus 
SELinux status:                 disabled
# facter selinux
Could not retrieve selinux: Invalid argument - /proc/self/attr/current

That means the fact $::selinux is later unavailable in Puppet and we should behave same way as if we get "false" status.