Until https://github.com/rr-debugger/rr/issues/2613
has been addressed, ./mtr --rr will fail to start up the
server if it has been configured with liburing.
To make the ./mtr --rr option work out of the box, we will
disable native asynchronous I/O.

Note: libaio never worked under rr, but it failed more gracefully:
the emulated io_setup() call would always return an error.