mlockall always applied by default without graceful failure

[douglas-raillard-arm]

The "lock_pages" JSON option is currently documented as applying to rt and deadline classes, but ignored for CFS. The actual behavior is that it's also applied for CFS tasks unless explicitly disabled.

A more reasonable behavior would probably be to enable it by default whenever possible, and when the user specifies something, apply it regardless of the scheduling class or the actual privileges (it's up to the user to come up with a conf file that works, and to rt-app to shout and exit with an error if that's not possible). This would make "lock_pages" available for CFS as well.