tools/io_uring: sync with liburing

Various fixes and changes have been applied to liburing since we
copied some select bits to the kernel testing/examples part, sync
up with liburing to get those changes.

Most notable is the change that split the CQE reading into the peek
and seen event, instead of being just a single function. Also fixes
an unsigned wrap issue in io_uring_submit(), leak of 'fd' in setup
if we fail, and various other little issues.

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