nspawn,man: use a common vocabulary when referring to selinux security contexts

nspawn,man: use a common vocabulary when referring to selinux security contexts

Let's always call the security labels the same way:

  SMACK: "Smack Label"
  SELINUX: "SELinux Security Context"

And the low-level encapsulation is called "seclabel". Now let's hope we
stick to this vocabulary in future, too, and don't mix "label"s and
"security contexts" and so on wildly.