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sio: use kern.ipc.somaxconn for listen() on Mac

listen() on Mac used to take SOMAXCONN as the backlog size. It is
just 128, which is too small when connections are incoming too
fast. They get rejected.

Increase of the queue size wasn't possible, because the limit was
hardcoded. But now sio takes the runtime limit from
kern.ipc.somaxconn sysctl setting.

One weird thing is that when set too high, it seems to have no
effect, like if nothing was changed. Specifically, values above
32767 are not doing anything, even though stay visible in
kern.ipc.somaxconn.

It seems listen() on Mac internally might be using 'short' or
int16_t to store the queue size and it gets broken when anything
above INT16_MAX is used. The code truncates the queue size to this
value if the given one is too high.

Closes #8130

NO_DOC=bugfix
NO_TEST=requires root privileges for testing

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