Replace enum+union of local channel types with a vtable.

There's now an interface called 'Channel', which handles the local
side of an SSH connection-layer channel, in terms of knowing where to
send incoming channel data to, whether to close the channel, etc.

Channel and the previous 'struct ssh_channel' mutually refer. The
latter contains all the SSH-specific parts, and as much of the common
logic as possible: in particular, Channel doesn't have to know
anything about SSH packet formats, or which SSH protocol version is in
use, or deal with all the fiddly stuff about window sizes - with the
exception that x11fwd.c's implementation of it does have to be able to
ask for a small fixed initial window size for the bodgy system that
distinguishes upstream from downstream X forwardings.

I've taken the opportunity to move the code implementing the detailed
behaviour of agent forwarding out of ssh.c, now that all of it is on
the far side of a uniform interface. (This also means that if I later
implement agent forwarding directly to a Unix socket as an
alternative, it'll be a matter of changing just the one call to
agentf_new() that makes the Channel to plug into a forwarding.)

defs.h

@@ -53,6 +53,8 @@ typedef struct Frontend Frontend;
 
 typedef struct ssh_tag *Ssh;
 
+typedef struct Channel Channel;
+
 /* Note indirection: for historical reasons (it used to be closer to
  * the OS socket type), the type that most code uses for a socket is
  * 'Socket', not 'Socket *'. So an implementation of Socket or Plug