helpers/osinfo: add security lockdown type detection

[rafaeldtinoco]

This allows user to check '/sys/kernel/security/lockdown' status.

[rafaeldtinoco]

@grantseltzer this is the feature that will be added to tracee:

// kernel lockdown check

lockdown, err := helpers.Lockdown()
if err != nil {
	return err
}
if lockdown == helpers.CONFIDENTIALITY {
	return fmt.Errorf("kernel lockdown is set to 'confidentiality', can't load eBPF programs.")
}
if debug {
	fmt.Fprintf(os.Stdout, "OSInfo: Security Lockdown is '%v'\n", lockdown)
}

After we're able to compile tracee with libbpfgo HEAD.

It will fix: aquasecurity/tracee#295

[grantseltzer]

I'm not familiar with lockdown mode at all, but the content of my file looks like this:

[*] cat /sys/kernel/security/lockdown
[none] integrity confidentiality

Where do you find the schema for this file? Is it the same on every distribution?