gh-124761: add socket.SO_REUSEPORT_LB

[graingert]

• Issue: Add socket.SO_REUSEPORT_LB and use it in socket.create_server(reuse_port=True) and BaseEventLoop.create_server(reuse_port=True) #124761

[graingert]

this is only half of the work as the flag needs to be used when reuse_port=True is passed to various apis

[picnixz]

In the PR for SO_ORIGINAL_DST that I just reviewed, I didn't really feel the need to document it since we don't really document every constant (and it was better to have the constant included without going through a round of documentation review).

Here, since it's FreeBSD-only I think it's nice to document it (SO_ORIGINAL_DST is exposed assuming some headers, which would probably make its docs a bit messy I think). Since there's no conditional header, I appreciate the additional documentation.

Note: I haven't thought about it but we could have actually added some .. versionadded to the SO_* entries though for SO_ORIGINAL_DST.

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I'd also like @erlend-aasland's review on this one if possible.

[graingert]

@picnixz can I get a merge?

[picnixz]

Er... I'm not a core dev! we should ask @erlend-aasland or maybe you give me commit rights for this one

[picnixz]

(I'm still not a core dev, I still can't merge this one :()