sockets: Add error code for EPIPE (and ECONNABORTED on Windows)

[badeend]

WASI sockets currently has no error code corresponding to EPIPE. When writing to a closed/broken socket:

• POSIX systems return EPIPE
• Windows returns ECONNABORTED

After searching around, this Windows-specific discrepancy is well-known:

• Android ADB, Mio, and OpenJDK treat it as EPIPE.
• libuv and nginx treat it as ECONNRESET.

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WASI doesn't have a good solution for it now:

• By the time the OS reports EPIPE, previously acknowledged writes may have been lost and and the guest should be informed about that. So swallowing the error is not right.
• Mapping it to ECONNRESET isn't right either, beause that implies that the peer actively reset the connection.
• Mapping (or: leaving) it as ECONNABORTED isn't optimal either, because that error code is usually only returned by accept and sometimes connect on Unix-like systems.

I think this warrants a new error code. I've called it connection-broken. I expect wasi-libc to map this to EPIPE.

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Wasmtime currently maps EPIPE to ok(()). With the new error code in place, this test can be updated to consistently expect an error cross platform.