scx_layered: add HAS_BEEN_RENAMED matching rule

[etsal]

Add a rule that tests whether a task has been renamed using pthread_rename_np() or through /proc. This has two uses:

• Using task renames to identify specific tasks in a large application, without requiring modifications for that application.
• Preventing tasks from renaming themselves to match into a more privileged layer for a performance boost.

[htejun]

Hmm... this is rather trivial to defeat, so I'm not sure how meaningful this is. There are other gating mechanisms already (e.g. cgroup membership), wouldn't that be sufficient?

[likewhatevs]

I think this is more about covering a case that cmdjoin doesn't cover:

• I don't know the name of the threads I care about (or don't always know), but I do know that rename will be called on it.