Turn Backend into a sensible classoid.

Nearly every part of the code that ever handles a full backend
structure has historically done it using a pair of pointer variables,
one pointing at a constant struct full of function pointers, and the
other pointing to a 'void *' state object that's passed to each of
those.

While I'm modernising the rest of the code, this seems like a good
time to turn that into the same more or less type-safe and less
cumbersome system as I'm using for other parts of the code, such as
Socket, Plug, BinaryPacketProtocol and so forth: the Backend structure
contains a vtable pointer, and a system of macro wrappers handles
dispatching through that vtable.

be_nossh.c

@@ -10,7 +10,7 @@ const int be_default_protocol = PROT_TELNET;
 
 const char *const appname = "PuTTYtel";
 
-Backend *backends[] = {
+const struct Backend_vtable *const backends[] = {
     &telnet_backend,
     &rlogin_backend,
     &raw_backend,