Add new or missing socket constants on linux

[hauntsaninja]

We currently silence many errors from socket constants in stubtest, because the availability of some constants cannot be expressed in the type system (and probably also because historically we weren't always the best at specifying all platform differences exactly). However, this means we don't notice new additions.

On macOS, with python3.10:

λ python3.10 -m mypy.stubtest --custom-typeshed-dir ~/dev/typeshed socket --concise | grep 'is not present in stub'
socket.CAPI is not present in stub
socket.IP_RECVTOS is not present in stub
socket.PF_SYSTEM is not present in stub
socket.SYSPROTO_CONTROL is not present in stub
socket.TCP_KEEPALIVE is not present in stub
socket.TCP_NOTSENT_LOWAT is not present in stub
[...]

I can add PRs for these / check this for Linux. Filing this issue mainly as a reminder for myself to do so. Would be good if someone wants to take another look at Windows.

[AlexWaygood]

I'm in favour of adding these, and of getting some more coverage from stubtest. I can take a look on Windows to see what needs adding.

Missing socket constants have also been requested a few times by users:

• #7077
• #5696

[AlexWaygood]

Stubtest is enabled for all the socket constants on Windows and macOS these days, but we still have two very broad allowlist entries on linux:

https://github.com/python/typeshed/blob/18cd196109938b690a97dc1dadf1c92b391b9639/tests/stubtest_allowlists/linux.txt#L1 https://github.com/python/typeshed/blob/18cd196109938b690a97dc1dadf1c92b391b9639/tests/stubtest_allowlists/linux.txt#L6