* Enable virtual DNS by default.
* Improve handling of header size:
This fixes an issue in UDP, where it would not take the header size into account when deciding how much to read.
* IPRAW socket. Fix an issue where the header would include "our" IP Address.
The doc is not very clear 5.2.3: they say "destination". But for consistency with UDP (5.2.2) where it seems clear that this is the peer's ip, we replace with the "destination" of the socket, or the "source" of the packet.
* minor tweak: signal with SOCK_SYNSENT that a connection is being established.
* remove Socket::myErrno as it serves no purpose.
It is a duplicate of (sn_sr == W5100_SN_SR_SOCK_SYNSENT).
* add some logging when reading/writing on a closed socket.
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.
