[unix] POSIX Message Queue support

[Elliot Robinson]

We've had a request for info regarding message queue support in `unix` [0]. The related headers are currently listed in `System/Posix.hs` under the "Unsupported interfaces" category. Is the community aware of a reason that we have chosen not to support this interface going forward, or is this an artifact from when the library was originally ported (late 2002 by Simon Marlow) which we can now ignore?

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Elliot Robinson
GPG Key: 9FEDE59A

 [0] https://github.com/haskell/unix/issues/47