fix: do not keep retrying accept if handshake fails for "proxy" type sockets

[0x804d8000]

In socketFinalizeAccept, if the handshake (magic validation) fails, we should return failure for "proxy" sockets.

When ncclProxyService tries accepting new connection, with the current behavior, a spurious connection can cause ncclSocketAccept endlessly burns CPU cycles. As ncclProxyService marks the listening socket non-blocking, once a spurious connection cause socketFinalizeAccept to fail, we immediately retry ncclTryAccept, and get EAGAIN, and retry ncclTryAccept...

Unfortunately we can't just change the default behavior though. Several other callsites rely on ncclSocketAccept to ignore spurious connections for them, e.g., callsites in bootstrap.c. IMHO, those callsites should be refactored and either 1) do retries themselves, or 2) use something new such as ncclSocketAcceptBlockUntilHandshakeSucceeds.

[0x804d8000]

Hi, @sjeaugey would you mind taking a look at it?