[kazu-yamamoto]

On Windows, socketPair emulates socketpair() with an AF_UNIX listener and connection. Windows' AF_UNIX misses a peer's closesocket which races a recv entering its wait: the recv never returns, though a second recv on the same socket gets EOF at once. It happens in plain C too, with blocking, overlapped and IOCP receives alike (a few to a few dozen times in 2000 rounds on windows-latest), and never over TCP. The non-threaded RTS hits it in a Haskell program; it was found through simple-sendfile's test suite.

This PR builds the pair over TCP on 127.0.0.1 instead:

• The listener is bound to an ephemeral loopback port, and the connection accepted is checked to be ours, since another process may connect to the port first.
• Both ends get TCP_NODELAY, as a stream pair should not delay small writes.
• The temporary file the AF_UNIX pair left behind goes, and so does the temporary dependency on Windows.

The pair's family is now AF_INET on Windows; the arguments were already ignored there.

The socketPair tests now run on Windows too, with a new one where the peer closes while the other end waits in recv. macOS, and Windows with GHC 9.8.2 and 9.12 (MIO and WinIO), pass.

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