Using core/sys/posix/mqueue.d on FreeBSD

[Jonathan M Davis]

Actually, since I'm usually the one who does the FreeBSD ones anyway, here
you go:

https://github.com/dlang/dmd/pull/16359

The declarations compile, and they should match the ones in C, since I
copied them over and then tweaked them, but I haven't actually tested them.

All that being said, even if they're merged immediately, they won't be
available as part of druntime until dmd 2.109.0 is released (and 2.108.0 was
released less than a week ago), so you'll probably need to copy them into
your own install or use the development version of dmd to get the updated
bindings if you want to use them now.

- Jonathan M Davis