. tfe.cpp renamed to Uthernet1.cpp.
. add class NetworkBackend: common to both U1 and U2 cards, and abstracts Windows/Linux backends.
. modernise error message if WPCAP.DLL is not installed.
* Make Uthernet settings behave like all other cards.
Fix as well the fact that the Uthernet interface would not be reapplied after a restart.
* Uthernet: use consistent types to reduce code complexity.
Use std::string everywhere.
Andrea