io_uring: flag new native workers with IORING_FEAT_NATIVE_WORKERS

A few reasons to do this:

- The naming of the manager and worker have changed. That's a user visible
  change, so makes sense to flag it.

- Opening certain files that use ->signal (like /proc/self or /dev/tty)
  now works, and the flag tells the application upfront that this is the
  case.

- Related to the above, using signalfd will now work as well.

Signed-off-by: Jens Axboe <axboe@kernel.dk>