ETH_P_* constants should be c_ushort

[AidanWoolley]

ETH_P_* constants (and probably more I haven't stumbled across) should be defined as c_ushorton linux x86-64 rather than c_int for 2 related reasons:

1. To improve ease of use with the various sockaddr_* structs where they most often get used.
2. Because when they are converted to big endian network ordering for use in those structs, (0x00000003).to_be() != (0x0003).to_be(), which is a really irritating gotcha.

I'm happy to do a find-and-replace to get those (and any others which people think should be changed) if you think it's a sane change.

[tgross35]

This is similar to #1619, something that is int in function signatures but short in structs. We'll have to figure out what to do here.