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.

ldisc.h

@@ -10,8 +10,7 @@
 
 struct Ldisc_tag {
     Terminal *term;
-    Backend *back;
-    void *backhandle;
+    Backend *backend;
     void *frontend;
 
     /*