Inconsistencies between Ethernet libraries of different architectures

I'm working on a multi architecture project and foundthat there are inconsistencies of Ethernet libraries for different architectures. E.g.:

1. Ethernet library for AVR contains Dns.h (with more DNS related functions) while Ehternet for Opta doesn't
2. Ehternet for AVR doesn't contain hostname related functions (e.g.: Ethternet.setHostName())

It'd be good to synchronize different Arduino architectures at API level, to make it possible to create portable code.

Compare the Ethernet libraries for AVR and Opta, starting with the APIs named in the issue: Dns.h, DNS functions, and hostname functions such as Ethernet.setHostName(). Define the common API surface and verify that the resulting interfaces support portable code across the architectures.

Written by the indexing model from the issue text.