Add support for mq_notify() on Linux

[ChrysoliteAzalea]

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What does this PR do

This PR adds support for the mq_notify() API that allows a process to register for POSIX message queue notifications.

Checklist:

• [yes] I have read CONTRIBUTING.md
• [yes] I have written necessary tests and rustdoc comments
• [yes] A change log has been added if this PR modifies nix's API