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.

ssh.h

@@ -629,7 +629,7 @@ extern const char sshver[];
  * that fails. This variable is the means by which scp.c can reach
  * into the SSH code and find out which one it got.
  */
-extern int ssh_fallback_cmd(void *handle);
+extern int ssh_fallback_cmd(Backend *backend);
 
 void SHATransform(word32 * digest, word32 * data);