In commit 5e62b6a5 (MDEV-16264)
the logic of os_aio_init() was changed so that it will never fail,
but instead automatically disable innodb_use_native_aio (which is
enabled by default) if the io_setup() system call would fail due
to resource limits being exceeded. This is questionable, especially
because falling back to simulated AIO may lead to significantly
reduced performance.

srv_n_file_io_threads, srv_n_read_io_threads, srv_n_write_io_threads:
Change the data type from ulong to uint.

os_aio_init(): Remove the parameters, and actually return an error code.

thread_pool::configure_aio(): Do not silently fall back to simulated AIO.

Reviewed by: Vladislav Vaintroub