Support POSIX Message Queue

Thanks for creating the great work first!

And now I am trying to replace some code with rustix APIs, in my project related to POSIX message queue (APIs: `mq_open`, `mq_send`, `mq_unlink`, etc.). However, it seems that current rustix does not support it, so I am sending this issue (feature request) and hope we can use POSIX message queue with rustix!

See https://man7.org/linux/man-pages/man7/mq_overview.7.html