feat(modbus): use poll() for Linux instead of select()

[fcntlcn]

The 1024 limitation of select() makes some problem these days. I think using poll() for *nix systems may be a better choice.

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[stephane]

After a quick look, it seems the tv struct could be replaced by a single integer var (ms). Are you sure it's supported on other Unix systems (FreeBSD, MacOS, etc)?