Uthernet II: add extended feature to virtualise DNS requests.
. This allows pure TCP/UDP sockets to run *without* MACRAW requests (and so without libpcap).
. Raw sockets will not work.
. Add configuration for Virtual DNS.
libpcap: ensure all functions check if the library is loaded before using it.
Uthernet 1: do NOT overwrite tfe_cannot_use as it should only reflect the availability of npcap on *this* system.
Add Copyright notice, and mention Virtual DNS in html.
. 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.
Andrea