compatibility for esp8266

[d-a-v]

This pull request:

• Fixes a warning in example compilation
• Allows to use local definitions for ntohs, htons, ntohl and htonl
• In case of esp8266 and because of changes in IPAddress to allow IPv6, uses standard IPAddress API to get raw IP address.

About IPAddress, the proposed change is portable with current arduino API. raw_address() would also be a another portable candidate relying on standard IPAddress.

Side note: this Ethernet library is planned to be integrated as a git submodule into esp8266 arduino core repository and replace the old one that was copied and locally patched.

[d-a-v]

@PaulStoffregen I will close this pull request per https://github.com/arduino-libraries/Ethernet/pull/96#issuecomment-555409264.

I express here my misunderstanding about the link above. I would be pleased to know more about it. I don't know which repository is better, I will use the other one given the facts that it is supposed to be the official one, they just merged the very same pull request, and you are supposed to have write access on it.