Don't apply the FD_SETSIZE check to Windows sockets

[StalderT]

On Windows a SOCKET is an opaque handle, not a small index, and an fd_set is a counted array of sockets rather than a bitmask. libmodbus only ever puts a single socket in a set, so it can't overflow it.

The fd >= FD_SETSIZE checks added in 7b13341 therefore don't apply on Windows, where they instead reject any socket whose handle value reaches FD_SETSIZE (64 by default) - which happens after a few dozen open handles and breaks Modbus TCP with EINVAL / "exceeds FD_SETSIZE".

Wrap the check in a macro that keeps it on POSIX and disables it on Windows. POSIX behaviour is unchanged.

Fixes #842

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