WSAPoll returns WSAEINVAL when there are no valid sockets in the fdarray
parameter. Individual WSAPOLLFDs can be ignored by setting them to a
negative value (just as with POSIX poll(2)), but there must be at least
one valid (not-ignored) socket to check.
Handled this by either returning error if the error was not EINVAL, or
by checking each WSAPOLLFD in the fdarray; at the first valid one,
return the error, since this was not the reason for the error. If none
are valid, continue, ignoring the error.
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